Bubba Mabry: private investigator in Albuquerque, New Mexico, by Steve Brewer

Caroline Mabry: police detective in Spokane, Washington, by Jess Walter

Marti MacAlister: widowed black police detective in Lincoln Prairie, Illinois, by Eleanor Taylor Bland

Frank MacAllister: swashbuckling private investigator in Brazi, by Ritchie Perry

Ian MacArthur: by Selwyn Jepson

Elsie "Beanie" MacBean: freelance writer and member of the Washoe Tribe, and her basset hound, Cruiser, in South Tahoe, Nevada, by Sue Owens Wright

Hamish Macbeth: police constable in Scotland, by M.C. Beaton

Stacey MacBeth: cop in Chicago, Illinois, by Dave Case

Grace MacBride: founder of Monkeewrench, a game software company, in Minneapolis, Minnesota, by P.J. Tracy

Liss MacCrimmon: 20-something dancer forced into early retirement by a knee injury, helping at her aunt’s Scottish store, in Moosetookalook, Maine, by Kaitlyn Dunnett (Kathy Lynn Emerson)

Devon MacDonald: private investigator in St. Paul, Minnesota, by Nancy Baker Jacobs

Paul MacDonald: ex-reporter and mystery writer, in San Francisco, California, by Julie Smith

Robert MacDonald: London Scot, Inspector and later Superintendent, in England, by E.C.R. Lorac (Carol Carnac)  

Dr. Duncan A. MacDonell: veterinarian in Houston, Texas, by Robert E. Armstrong

Frankie MacFarlane: geologist based in Tucson, Arizona, by Susan Cummins Miller

Fiona MacGillivray: owner of the Savoy dance hall, her 12-year old son Angus, and constable Richard Sterling, in the gold rush of 1898, in Dawson, Yukon Territory, Canada, by Vicki Delany

Maggie MacGowen: documentary filmmaker in Los Angeles, California, by Wendy Hornsby

Will MacGowen, whose heroin addiction cost him his job as narcotics detective with the LAPD, now police chief in the fictional rural town of Haydenville, California, by Ken Mercer

Kevin MacInnes: the “Lie King”, a free-lance lie detector and private investigator, in Ohio and elsewhere, by Franklin Bandy

Isabel Macintosh: faculty dean, and Giles Sullivan, a retired attorney, in Vermont, in the Crossword Puzzle mysteries by Herbert Resnicow

Kathryn Mackay: prosecuting attorney in northern California, by Christine McGuire

Scotia MacKinnon: former police officer turned private eye in Friday Harbor, San Juan Island, Washington, by Sharon Duncan

Brett Macklin: aka Mr. Jury, aka the .357 Vigilante in Los Angeles, California, by Ian Ludlow (Lee Goldberg)

Madeline “Mac” Maclin: divorced former local beauty queen, and struggling private investigator, in Celosia, North Carolina, by Jane Tesh

Peter Macklin: hit man for the mob in Detroit, Michigan, by Loren D. Estleman

Herbert Mackey: sheriff, and John Marlin, game warden, in Blanco County, Texas, by Ben Rehder

Captain Duncan Maclain: blinded by gas in WWI, working as a detective, assisted by his wife Rena, and Spud Savage, in New York City, by Baynard Kendrick

Kate MacLean: police detective in the Seattle, Washington area, by Noreen Gilpatrick

Theresa MacLean: forensic scientist, in Cleveland, Ohio, by Lisa Black (Elizabeth Becka)

Cooper MacLeish: taxi driver and Vietnam vet, in Chicago, Illinois, by Sam Reaves

Chanse MacLeod: gay private investigator in New Orleans, Louisiana, by Greg Herren

Enzo Macleod: Scottish biologist based in France, by Peter May

Dr. Rhona MacLeod: forensic scientist in Glasgow, Scotland, by Lin Anderson

Declan (Mac) MacManus: struggling 30-something part-time private investigator living above a bar, in Southampton, New York, by Daniel Judson

Dusky MacMorgan: ex-Navy SEAL, in Florida, by Randy Striker (Randy Wayne White)

Harry MacNeil: ex-cop private investigator who wants to be a jazz clarinetist, in 1930s New York City, by H. Paul Jeffers

Elisha Macomber: 70-something Chairman of the Board of Selectmen of Penberthy Township, and proprietor of a fish market, in fictional Penberthy Island, Massachusetts, by Kathleen Moore Knight

Camilla MacPhee: who runs an advocacy agency for victims of violent crime, in Ottawa, Ontario, Canada, by Mary Jane Maffini

Annie MacPherson: attorney in Seattle, Washington, by Janet Smith

Elizabeth MacPherson: forensic anthropologist in southern USA, by Sharyn McCrumb

Alonzo MacTavish: debonair rogue in England, by Peter Cheyney

David Madden: postal inspector, by Doris Miles Disney

Inspector John Madden: of Scotland Yard, a shell-shocked veteran of World War I, by Rennie Airth

Ivor Maddox: sergeant at the Wilcox Avenue Station in Hollywood, California, by Elizabeth Linington

Madigan: police lieutenant in New York City, by Edwin Lanham

Paul Madriani: defense attorney in California, by Steve Martini

Nick Madrid: freelance journalist, by Peter Guttridge

Nick Magaracz: private eye in Trenton, New Jersey, by Kate Gallison

Philip Magellan: the Marksman, dedicated to wiping out the Mafia, by Peter McCurtin

Gianna Maglione: lesbian police lieutenant, and Mimi Patterson, a reporter, in Washington, DC, by Penny Mickelbury

Blue Maguire and Spaceman Kowalski, Vietnam vets and cops, in Los Angeles, California, by Teri White

Cormac Maguire: Irish archaeologist, and Nora Gavin, an American pathologist, in Ireland, by Erin Hart

Maggie Maguire: ex-Pinkerton agent in 1870s San Francisco, California, by Kate Bryan

Ambrose Mahon: police inspector, later superintendent, in Australia, by S.H. Courtier

Sandra Mahoney: government clerk, later a computer security consultant, in Canberra, Australia, by Dorothy Johnston

Inspector Maigret: police inspector in Paris, France, and Madame Maigret, his wife, by Georges Simenon

Anthony Maitland: barrister in England, by Sara Woods

Evan Maitland: former police detective, now running an antique business and moonlighting as a bounty hunter, based in Chicago, Illinois, by James Patrick Hunt

Kyle Malachi: who publishes mystery book reviews under the pseudonym Stokes Moran, by Neil McGaughey

Richard Malcolm: CIA analyst and grad student, in Washington, DC, by James Grady

Casey Maldonado: trying to come to terms with loss, and her companion Death, the Grim Reaper, by Judy Clemens

Steve “Snap” Malek: police reporter for the Tribune, in 1930s–1940s Chicago, Illinois, by Robert Goldsborough

Lord Ambrose Malfine: English aristocrat back from fighting the Greek War of Independence, by Jane Jakeman

Don Mallett: thief and poacher, in Medwell Fratorum, England, by Frank Parish (Roger Erskine Longrigg)

Inspector Mallett: in England, by Cyril Hare

William Mallett: British secret service agent, by Douglas Orgill

David Mallin and sometimes George Coe, private detectives in England, by Roger Omerod

Mallory: small-town student mystery writer in Iowa, by Max Allan Collins

Blair Mallory: health club owner in North Carolina, by Linda Howard

Dev Mallory: Secret Service Agent in the post-Civil War US, by Ed Gorman

Kathleen Mallory: police detective in New York City, by Carol O’Connell

Stuart Mallory: private investigator in Lansing, Michigan, by Ronald Clair Roat

Chance Malloy: wealthy, 39-year-old bachelor running an aviation business in South America, by Lester Dent

Claire Malloy: small-town bookstore owner in Farberville, Arkansas by Joan Hess

Hannah Malloy and Kiki Goldstein, an unlikely pair of 60-something sisters in Marin County, California, by Annie Griffin

Vic Malloy: private investigator in California, by James Hadley Chase

Cotton Malone: ex-US Justice Department agent, by Steve Berry

Fritz Malone: private investigator, and bastard son of a former police commissioner, in New York City, by Richard Hawke (Tim Cockey)

Ivy Malone: little old lady who uses her inconspicuousness in her sleuthing, by Lorena McCourtney

John J. Malone: lawyer in Chicago, Illinois, by Craig Rice

Kenneth Malone: in then-future 1970s New York City, by Mark Phillips (joint pseudonym of Randall Garrett & Larry M. Harris)

Ryder Malone: private investigator in Jersey City, New Jersey, by J.W. Rider (Shane Stevens)

Scobie Malone: family-man and a police inspector, in Sydney, Australia, by Jon Cleary

Sheila Malory: writer of literary criticism in Taviscombe, England, by Hazel Holt

Augustus Maltravers: journalist turned playwright and novelist, in fictional Vercaster, England by Robert Richardson

George Man: 18th-century parish watchman in London, England, by Keith Heller

Bolivar Manchenil: police captain, and grandson of a freed slave, on an unnamed island in the Caribbean, by Donald McNutt Douglass

Fu Manchu: pursuer of world dominance, by Sax Rohmer

Munch Mancini: ex-druggie and jailbird in 1970s Los Angeles, California, by Barbara Seranella

Dante Mancuso: private investigator, AKA the Pelican, in the North Beach area of San Francisco, California, by Domenic Stansberry

Augustus Mandrell: of the one-man professional killer firm Mandrell, Ltd., during and after WWII, by Frank McAuliffe

Cuddy Mangum and Justin Savile, police detectives in a small town in North Carolina, by Michael Malone

Terry Manion: private investigator in New Orleans, Louisiana, by Dick Lochte

Tiger Mann: spy in New York City, by Mickey Spillane

Mark Manning: gay journalist in Chicago, Illinois, by Michael Craft

Joe Mannix: private investigator in Los Angeles, California, in “Mannix” TV series novelizations, by J.T. MacCargo (Peter Rabe)

Patrick Mansfield: detective sergeant, and James Roland, a detective inspector, in Suffolk, England, by Ann Quinton

Dr. Manson by Edwin Radford and Mona Augusta Radford

Inspector Manson by Edwin Radford and Mona Augusta Radford

Simon Manton: police inspector, later superintendent, in London, England, by Michael Underwood

Paul Manziuk: Detective Inspector with homicide, and Jacqueline (Jackie) Ryan, a black cop recently promoted to Detective, in Toronto, Ontario, Canada, by N.J. Lindquist

David Mapstone: ex-cop, now unemployed college history teacher back in law enforcement, in Phoenix, Arizona, by Jon Talton

Sir Geoffrey Mappestone: knight during the time of the Crusades, by Simon Beaufort

Aurelia Marcella: young Roman innkeeper in 1st century Roman Britain, by Jane Finnis

Colonel March: head of Scotland Yard’s fictitious Department D-3, the Department of Queer Complaints, by Carter Dickson

John March: black sheep of a staid merchant-banking family, now an ex-deputy sheriff working as a private investigator in Manhattan, New York City, by Peter Spiegelman

Tobias March and Lavinia Lake, lovers and partners-in-crime investigators in Regency London, England, by Amanda Quick

Daniel Marchant: suspended MI-6 agent trying to clear his father’s reputation as former head of MI-6, in the globetrotting Legoland (MI-6 headquarters) trilogy by Jon Stock

Josie Marcus: undercover store spy in St. Louis, Missouri, in the Mystery Shopper series by Elaine Viets

Tom Mariner: detective inspector in Birmingham, England, by Chris Collett

Lili Marino: moving from Brooklyn to Walden Corners, New York, in the Gourd Craft mysteries by Maggie Bruce

Bascot de Marins: Templar Knight recovering from imprisonment in the holy lands, in the early 1200s, in England, by Maureen Ash

Alan Markby: detective inspector, and Meredith Mitchell, a Foreign Service officer, in the Cotswolds, England, by Ann Granger

Helen Markham-Johnson: actress recruited by the Secret Service in London, England, by Eileen Dewhurst

Peter Marklin: toyshop owner and antique toy collector in Dorset, England, by Neville Steed

Lieutenant Marks: detective in New York City, by Dorothy Salisbury Davis

Calista Marley, a forensic pathologist, and Plato Marley, a family physician, in Cleveland, Ohio, by Bill Pomidor

John Marlin: game warden, and Herbert Mackey, a sheriff, in Blanco County, Texas, by Ben Rehder

Inspector Harvey Marmion and Sergeant Joe Keedy: metropolitan police in 1915 London, England, by Edward Marston

Jake Maroc: top agent for the Quarry, a secretive US government agency, by Eric Van Lustbader

Mallory Marlowe: recent widow with a new career as a travel writer, by Cynthia Baxter

Philip Marlowe: private eye on the mean streets of Los Angeles, California, by Raymond Chandler

Raymond Chandler’s Philip Marlowe, a private eye in Los Angeles, California by Robert B. Parker

Pru Marlowe: animal psychic returning to her hometown in the Berkshires, New York, by Clea Simon

Ursula Marlowe: Oxford-educated heiress, suffragette, and aspiring journalist in Edwardian London, England, by Clare Langley-Hawthorne

Miss Marple: spinster living in St. Mary's Mead, England, by Agatha Christie

John Marquez: an ex-DEA agent, now an undercover warden with the California Department of Fish and Game, by Kirk Russell

Stevie Marriner: soap opera star, and former spouse Neil Gulliver, former newspaper crime reporter, in Los Angeles, California, by Robert S. Levinson

Stephen Marryat: attorney in England, by Margaret Leek

Maggie Mars: struggling screenwriter and former investigative journalist, in Los Angeles, California, by Cady Kalian

Cat Marsala: freelance investigative journalist in Chicago, Illinois, by Barbara D’Amato

Grace Marsden: writer of children’s books in Pine Marsh, Illinois, by Luisa Buehler

Harry Marsh: chief inspector, and Dr. Clio Rees, a physician and mystery writer, in England, by Jo Bannister

Jennifer Marsh: murder mystery writer in Georgia, by Judy Fitzwater

Peter Marsh: wheelchair-bound ex-policeman, and his daughter Georgia, who investigate unsolved murders in Kent, England, by Amy Myers

Alec Marshall: British army major making inquiries into the will of Cecil Rhodes, at the time of Edward VII’s coronation, in 1902 London, England, by Rafe McGregor

Neil Marshall: graduate student in creative writing, struggling poet, and part-time chef and caterer, in Houston, Texas, by Tim Hemlin

Harriet Martens: detective superintendent in Greater Birchester, England, by H.R.F. Keating

Eduard Martíez and Borja Masdéu, non-identical twins and partners in Frau Consultants, an extralegal investigative agency, in Barcelona, Spain, by Teresa Solana

Alessandra (Al) Martillo: working as an assistant to Marty Stiles, an ex-NYPD cop turned PI, by Norman Green

Bernard Martin: investigating magistrate in late 19th century Provence, France, by Barbara Corrado Pope

Cézanne Martin: cop turned lawyer in Fort Worth, Texas, by Laurie Moore

Christine Martin: 40-something cub reporter in England, by Annette Roome

Dorothy Martin: American schoolteacher retired in England, by Jeanne M. Dams

George Martin: police inspector, in England, by Francis Beeding

Holly Martin: corporal in the Royal Canadian Mounted Police, on Vancouver Island, British Columbia, Canada, by Lou Allin

Lizzie Martin: companion to a wealthy widow slum landlord, in 1860s London, England, by Ann Granger

Pepper Martin: tour guide at a cemetery, who can talk with a mob boss ghost, in Cleveland, Ohio, by Casey Daniels

Rina Martin: retired actress, who played a private investigator in a television series, in Frantham, England, by Jane Adams

Savannah Martin: real estate agent and southern belle, in Nashville, Tennessee, by Bente Gallagher (Jennie Bentley)

Saz Martin: lesbian private investigator in London, England, by Stella Duffy

Thomas Martindale: journalism professor at Oregon State University, by Ron Lovell

Steve “Two Crows” Martinez: Lakota Indian by birth, eastern white by upbringing, and deputy sheriff in Porcupine City, in the Upper Peninsula of Michigan, by Henry Kisor

Harry Martineau: detective inspector, later Detective Chief Inspector, in fictional Granchester, “Metropolis of the North,” England, by Maurice Procter

Kate Martinelli: lesbian homicide detective, and homicide detective Alonzo Hawkin, in San Francisco, California, by Laurie R. King

Rebecka Martinsson: tax attorney in Stockholm, called back to her hometown Kiruna, north of the Arctic Circle, in Sweden, by Åsa Larsson

Paul Martiny: accountant exacting revenge against the privileged, in Cyprus and England, by William Haggard

Frank Marvelli and Loretta Kovacs, members of a parole violators search team in New Jersey, by Anthony Bruno

Albie Marx: ex-60s radical and a columnist for a radical magazine in 1990s Los Angeles, California, by David Debin

Groucho Marx: movie star in Hollywood, California, by Ron Goulart

Borja Masdéu and Eduard Martíez, non-identical twins and partners in Frau Consultants, an extralegal investigative agency, in Barcelona, Spain, by Teresa Solana

Alan Mason: police superintendent, and his sidekick Ian Crawford, based in the town of Greyhavens, Scotland, by Francis Lyall

Perry Mason: defense attorney in Los Angeles, California, by Erle Stanley Gardner

Lou Mason: attorney in Kansas City, Missouri, by Joel Goldman

Tom Mason: gay teacher, and his lover, Scott Carpenter, a baseball player, in Chicago, Illinois, by Mark Richard Zubro

Winter Massey: deputy U.S. Marshal (later an ex-marshal), in New Orleans, Louisiana, and elsewhere in the South, by John Ramsey Miller

George Masters: Scotland Yard DCI and later Chief Superintendent, and Bill Green, a Detective Inspector and later DCI, in London, England, by Douglas Clark

Molly Masters: cartoonist and greeting card entrepreneur, in Albany, New York, by Leslie O’Kane

Masao Masuto: Detective Sergeant in Beverly Hills, California, by E.V. Cunningham

Angela Matelli: ex-Marine turned private investigator, in Boston, Massachusetts, by Wendi Lee

Mathilde of Westminster: physician and former lady-in-waiting, in the court of Edward II, in 14th century, London, England, by Paul Doherty

Jana Matinova: police commander in Bratislava, Slovakia, by Michael Genelin

Gunnar Matson: police sergeant in San Francisco, California, by Brèni James

Charles Matthews: sheriff in Crawford County, Texas by D.R. Meredith

Daphne Matthews: police psychologist, and Lou Boldt, a detective, in Seattle, Washington, by Ridley Pearson

Margaret Ann Matthews, Bonnie Lynn Tyler, and Lindsey Fox, cousins at Pinckney Plantation on fictional Indigo Island, South Carolina, by Caroline Cousins

Nell Matthews: newspaper reporter, and Mike Svenson, a police officer, in Grantham, Oklahoma, by Eve K. Sandstrom

Whiskey Mattimoe: recently widowed, 30-something real estate broker, in the small resort town of Magnet Springs, Michigan, by Nina Wright

H.J. Mavity and Ben Spanner, ex-spouses in Connecticut, by Ron Goulart

Alex Mavros: half-Greek, half-Scottish private investigator in Greece, by Paul Johnston

Gaston Max: in London, England, by Sax Rohmer

Harry Maxim: Special Services major, assigned to 10 Downing Street in London, England, by Gavin Lyall

Georgia Lee Maxwell: disaffected Florida society editor who moves to Paris to write a magazine column, in Paris, France, by Mickey Friedman

Lauren Maxwell: naturalist and investigator for the Wild America Society in Anchorage, Alaska, by Elizabeth Quinn

“Mama” Maxwell and Janna Brill, detectives in a future Topeka, Kansas, by Lee Killough

Michelle Maxwell and Sean King, Secret Service agents, in Washington, DC, and Virginia, by David Baldacci

Peter Maxwell: widowed teacher and golden-hearted cynic, in England, by M.J. Trow

John May and Arthur Bryant, detectives in the Peculiar Crimes Unit, in London, England, by Christopher Fowler

Dr. Tina May: television celebrity pathologist and amateur sleuth, in England, by Sarah Kemp (Michael Butterworth)

Tom Maybridge: Detective Chief Inspector in England by B.M. Gill

Major Thomas Maynard: detective in 1875-76 Dakota and Montana Territory, by Donald Honig

Asey Mayo: former sailor and auto racer in Cape Cod, Massachusetts, by Phoebe Atwood Taylor

Gil Mayo: police detective in the Midlands, England, by Marjorie Eccles

Kerry and Terry McAfee: twin fledgling investigators in Southern California, by Jennifer Colt

Jo McAllister: proprietor of Jo’s Craft Corner, in the small town of Abbotsville, Maryland, in the Craft Corner mysteries by Mary Ellen Hughes

Alan McAlpine: detective inspector at Partickhill Station in Glasgow, Scotland, by Caro Ramsay

Ed McAvoy: police chief in Ulster County, New York, by Bill Stackhouse

Rex McBride: private eye in Los Angeles, California, by Cleve F. Adams

Cassidy McCabe: psychotherapist, and her calico cat Starshine, in Oak Park, Illinois, by Alex Matthews

Mike McCabe: former NYPD homicide detective, now a detective sergeant living with his 13-year-old daughter in Portland, Maine, by James Hayman

McCaig: detective inspector, later superintendent, in Scotland, by Hugh C. Rae

Jack McCain: ex-crime scene detective and forensic scientist, in Sydney, Australia, by Gabrielle Lord

Sam McCain: attorney in Black River Falls, Iowa, by Ed Gorman

Terry McCaleb: former FBI man recuperating from a heart transplant, in Los Angeles, California, by Michael Connelly

Kate McCall: a retiree and her band of amateur sleuths at Serenity Cove Estates, South Carolina, in the Bunco Babes mysteries by Gail Oust

Tom McCall: commercial photographer living on a houseboat, in Vancouver, British Columbia, Canada, in the Granville Island mysteries by Michael Blair

Duncan McCallum: Scottish prisoner transported to Colonial America, by Eliot Pattison

Tess McCallum: FBI special agent, and Abby Sinclair, a freelance security consultant who hunts stalkers, in Los Angeles, California, by Michael Prescott

Doan McCandler: gay private investigator. and Binky Van de Kamp, his sidekick. in San Francisco, California, by Orland Outland

Elias McCann: the reluctant community coroner, in Tofino, Vancouver Island, British Columbia, Canada, by Mark Zuehlke

Blue McCarron: independent social psychologist and private investigator living outside San Diego, California, by Abigail Padgett

Gail McCarthy: veterinarian in an equine practice, in Santa Cruz, California, by Laura Crum

Jay McCaulay: former captain in the Paras, now working with TRACE, tracking down missing children in war zones, by C.J. Carver

Quint McCauley: ex-cop turned private investigator in a suburb of Chicago, Illinois, by D.C. Brod

Rubens McCauley: maverick detective in the Criminal Investigation Unit in St. Kilda, a suburb of Melbourne, Australia, by Jarad Henry

Mike McCleary and Quin St. James, a wife and husband PI team in Florida, by T.J. MacGregor

Adam McCleet: ex-cop turned sculptor, in Portland, Oregon, by Rick Hanson

Marjorie McClelland: a smart and sassy mystery writer, and rich British expat Creighton Ashcroft, in 1930s Ridgebury, Connecticut, by Amy Patricia Meade

Lara McClintock: antiquities dealer and shop owner in Toronto, Ontario, Canada, in the Archeological mysteries, by Lyn Hamilton

Shirley McClintock: 50-something rancher in Colorado, by B.J. Oliphant

Wilton McCleary: Civil War veteran and policeman, in 1870s Philadelphia, Pennsylvania, by Mark Graham

McCloud: New Mexico marshal in New York by Collin Wilcox

Jack McClure: ATF agent and Special Advisor and closest friend to the President of the United States, by Eric Van Lustbader

Sharon McCone: legal investigator and private eye, in San Francisco, California, by Marcia Muller

Andi McConnell: who turns 60, loses her job, and inherits a limousine, by Lorena McCourtney

Lt. Kate McConnell: homicide detective, and Sam Kincaid, chief of the Special Investigations Branch of the Department of Corrections, in Salt Lake City, Utah, by Michael Norman

Doug McCool: Vietnam vet and process server in San Francisco, California, by Miles Archer

“Mac” McCorkle: saloon owner, and Mike Padillo, a spy, in Bonn, Germany, by Ross Thomas

Darby McCormic: holding a doctorate in criminal psychology and working as a crime scene investigator, in Boston, Massachusetts, by Chris Mooney

Sunny McCoskey: chef in wine country, Napa Valley, California, by Nadia Gordon

Blaine McCracken: exiled agent who knows 14 ways to kill a man in under two seconds, by Jon Land

James “Mack” McCray: federal undercover cop in Stuart, Florida, in the Treasure Coast mystery series, by Paul McElroy

Carrie McCrite: widow in the Ozark mountains of Arkansas, in the Something To Die For mysteries, by Radine Trees Nehring

Dickson Mc’Cunn: grocer, and the Gorbals Die-Hards, in Scotland, by John Buchan

Claude McCutcheon: laid-back bachelor lawyer in the San Francisco Bay Area, in California, by John A. Miller

Katie McDonald: mathematician and Sudoku whiz who returns to her hometown of Granville, New Hampshire, by Shelley Freydont

James McDougal: homicide inspector, and Lucy Ramsdale, a widowed artist, in Wingate, Connecticut, by Hildegarde Dolson

Jack McEvoy: reporter, and Rachel Walling, an FBI agent, in Denver, Colorado, and Los Angeles, California, by Michael Connelly

Korine McFaile, Janey Bascom, and Amilou Whittier, landscapers and partners in Three Dirty Women Landscaping, Inc., in Pine Grove, by Julie Wray Herman

Mac McFarland: burned-out reporter in Cape Cod, Massachusetts, by Doug Kiker

Peter McGarr: police officer in Dublin, Ireland, by Bartholomew Gill

Kirk McGarvey: CIA agent and assassin, later Director, by David Hagberg

Travis McGee: adventurer, philosopher, and private investigator, in Fort Lauderdale, Florida, by John D. MacDonald

Leonid McGill: black ex-boxer, old-school private investigator, in New York City, by Walter Mosley

Aubrey McGinty: cub reporter, and Dolly Madison (Maddy) Sprowls, 60-something newspaper archivist for the Herald-Union, in Hannawa, Ohio, in the Morgue Mama mysteries by C.R. Corwin

Sam McGowan: ex-cop in San Francisco, California, by Tim Maleeny

Nuala Anne McGrail: young Irish immigrant psychic and singer in Chicago, Illinois, by Andrew M. Greeley

Annie McGrogan: an actor and private investigator in New York City, by Gillian B. Farrell

Amos McGuffin: hard-drinking, golf-playing private investigator, based in San Francisco, California (also in Los Angeles and New York City), by Robert Upton

Joe McGuire: detective lieutenant in Boston, Massachusetts, who drops out to the Bahamas from time to time, by John Lawrence Reynolds

Regan McHenry: real estate agent in Santa Cruz, California, by Nancy Lynn Jarvis

Tom McInnes: young attorney in Coopers Bend, Alabama, by Mike Stewart

John McIntire: retired military intelligence officer and newly appointed township constable, in 1950s St. Adele, on the Upper Peninsula of Michigan, by Kathleen Hills

Urbino McIntyre: American expatriate writer and amateur sleuth, and his friend Barbara, Countess da Capo-Zendrini, in Venice, Italy, by Edward Sklepowich

Jenny McKay: 40-something reporter at the lowest rated TV station in New York City, by Dick Belsky

Austin McKenzie: veterinarian, and his wife Madeline, in Trumansburg in upstate New York, by Claudia Bishop

Greg McKenzie: retired Air Force criminal investigator in Nashville, Tennessee, by Chester Campbell

Sam McKall: political advisor in Utah, by Eric C. Evans

Biscuit McKee: librarian, and her cat, Marmalade, in Martinsville, a small town in northern Georgia, by Fran Stewart

Christopher McKee, known as the “Scotsman,” a police inspector, later head of Manhattan’s homicide squad, mostly in New York City, by Helen Reilly

Nora McKee: a college professor, and librarian Alex Kincaid, fighting shadowy government agents using time travel, by T.J. MacGregor

Charlie McKelvey: 30-year veteran police detective whose runaway son was murdered, starting in 1999 Toronto, Ontario, Canada, by C.B. Forrest

Brian McKenna: police detective in New York City, by Dan Mahoney

Owen McKenna: ex-San Francisco homicide inspector turned private investigator, and his Harlequin Great Dane, Spot, at Lake Tahoe, California, and Nevada, by Todd Borg

Michael McKenna: detective chief inspector in Wales, by Alison G. Taylor

Patience Campbell McKenna: 6-ft romance novelist turned crime writer, in New York, by Orania Papazoglou (Jane Haddam)

Alex McKenzie: a doctor in Maine,and Sarah Deane, a graduate student, by J.S. Borthwick

Ellie McKenzie: newly certified real estate agent returning to her old hometown of Santa Louisa, California, by Kathleen Delaney

Joanna McKenzie: former CID turned private investigator in Bath, England, by Margaret Duffy

Lydia McKenzie: an edgy art photographer who also works as an administrative assistant at the D’Angelo detective agency, in the Williamsburg section of Brooklyn, New York City, by Meredith Cole

Lee McKinney: divorcee who returns to Michigan to work for her aunt's chocolate business, in the Chocaholic mysteries, by Joanna Carl

Alex McKnight: former Detroit cop, now a private investigator running a hunting camp built by his late father, in Paradise Michigan on the shore of Lake Superior, by Steve Hamilton

Meg McLean: librarian, and Rob Neill, a sheriff’s investigator, in the Latouche County mysteries by Sheila Simonson

Mike McLean: freelance journalist in England, by Paul Adam

John McLeish, DCI, and Francesca Wilson, a civil servant turned housewife, in London, England, by Janet Neel

Catherine McLeod: investigative reporter in Denver, Colorado, by Margaret Coel

Michael McLoughlin: detective inspector, later retired, in Dublin, Ireland, by Julie Parsons

Liam McLusky: detective inspector transferred from Southampton to Bristol, England, by Peter Helton

Frank McMorran: detective inspector in Scotland, by Marten Claridge

Jack McMorrow: small-town editor in Androscoggin, Maine, by Gerry Boyle

Christina McMullen: cocktail waitress turned professional psychologist, and homicide detective Jack Rivera, in Los Angeles, California, by Lois Greiman

Maxine “Maxie” McNabb: 60-something Alaskan widow, exploring the USA in her Winnebago with her faithful companion, miniature dachshund Stretch, by Sue Henry

Archy McNally: playboy private eye in Palm Beach, Florida, by Lawrence Sanders

Charlotte “Charlie” McNally: 40-something TV investigative reporter, in Boston, Massachusetts, by Hank Phillippi Ryan

J. McNee: former cop, now private investigator, in Dundee, Scotland, by Russel D. McLean

Izzy McNeil: young entertainment lawyer in Chicago, Illinois, by Laura Caldwell

Brian McNulty: bartender in 1980s Manhattan, New York City, by Con Lehane

Sutton McPhee: reporter in Fairfax, County, Virginia, by Brenda English

Alan “Mac” McQueen: true-blue Australian intelligence agent, in Southeast Asia and Australia, by Mark Abernethy

Dennis McQueen: detective sergeant in New York City, by Robert J. Randisi

Rose McQuinn: daughter of Inspector Jeremy Faro, a detective in Victorian Edinburgh, Scotland, returning from the American Wild West, by Alanna Knight

Joyce McQuinnie: British-Australian teenage intern to C.F. Wong, feng shui master based in Singapore (later Shanghai, China), by Nury Vittachi

Logan McRae: detective sergeant in Aberdeen, Scotland, by Stuart MacBride

Stoner McTavish: lesbian travel agent in Boston, Massachusetts, by Sarah Dreher

Tish McWhinny: 70-something Vermont artist and painter, by B. (Barbara) Comfort

Lois Meade: who cleans houses for pin money in the quaint village of Long Farnden, England, by Ann Purser

Jayne Meadows and Steve Allen, celebrity crime-solvers in Los Angeles, California, by Steve Allen

Major Mearns and Sergeant Denny, special agents for William Pitt to watch George III, in England, by Gwendoline Butler

Richard Medford: inspector in the CID, based in England, by Holly Roth

John Medina: CIA agent, by Linda Howard

Patricia “Paddy” Meehan: crime reporter for the Scottish Daily News, in Glasgow, Scotland, by Denise Mina

Charlie Meiklejohn: ex-arson investigator private investigator, and Constance Leidl, a psychologist, in New York, by Kate Wilhelm

Susan Melville: spinster art teacher and painter, who becomes a freelance assassin, in New York City, by Evelyn E. Smith

Men at War Series: featuring the OSS during World War II by W.E.B. Griffin

Tony Mendez: Sheriff's detective in mid-1980s Oak Knoll, California, in the Deeper than the Dead series by Tami Hoag

Luis Mendoza: police lieutenant in Los Angeles, California, by Dell Shannon (Elizabeth Linington)

Miguel Menendez: Indio investigator for the Bureau of Tourism, in San Luis Potosi, Mexico, by Suzanne Blanc

Anne Menlo: child psychologist in Phoenix, Arizona, by Maxine O’Callaghan

Loren Mensing: law-school professor, in St. Louis, Missouri, Francis M. Nevins

Philip Mercer: engineer, geologist, and eco-warrior, by Jack Du Brul

Lord Meren: chief investigator for Pharaoh Tutankhamun in ancient Egypt, by Lynda S. Robinson

Jack Merlin: surgeon and magician, in Pittsburgh, Pennsylvania, by James Tucker

The Great Merlini: a stage magician, by Clayton Rawson

Michael Merrick: ,newspaperman in 1840s Boston, Massachusetts, by Mary Jo Adamson

Desmond Merrion: military intelligence officer who becomes a private detective, and Henry Arnold, a Scotland Yard inspector, in England, by Miles Burton (John Rhode)

Sir Henry Merrivale: holder of one of the oldest baronetcies in England, physician, barrister, and head of military intelligence for the war office in England, by Carter Dickson (John Dickson Carr)

Maggie Messina: owner of the Black Sheep Knitting Shop and a circle of knitters, in Plum Harbor, Massachusetts, in the Black Sheep Knitting mysteries by Anne Canadeo

Decius Cecilius Metellus: a would-be playboy son of an illustrious family, in the SPQR historical mysteries, by John Maddox Roberts

Sofie Metropolis: Greek-American private investigator, in Queens, New York City, by Tori Carrington

Hazel Micallef: 60-something detective inspector in the small town of Port Dundas, Ontario, Canada, by Inger Ash Wolfe

Ian Michaels: Silicon Valley entrepreneur, in Palo Alto, California, in the Silicon Valley mysteries by Keith Raffel

Laura Michaels: retirement center social worker in Timberlake, Oklahoma, by Jack Bickham (John Miles)

Richard Michaelson: a retired Foreign Service Office diplomat, and Marjorie Randolph, a bookstore owner, in Washington, DC, by Michael Bowen

David Middleton-Brown: solicitor, and Lucy Kingsley, an artist in London, England, and the Church of England, in the Book of Psalms mysteries by Kate Charles

Garreth Mikaelian: vampire cop, first in San Francisco, California, and later in Baumen, Kansas, by Lee Killough

John Milano: private investigator, by Stanley Ellin

Robert (Bobby) Miles: minor league baseball player turned private investigator and troubleshooter for the Speaker of the Illinois House of Representatives, by David Everson

Tricia Miles: running a mystery bookstore, Haven’t Got a Clue, and the cat Miss Marple, in fictional small-town Stoneham, New Hampshire, a town full of bookstore, by Lorna Barrett (L.L. Bartlett)

Alec Milius: young college graduate recruited by British intelligence (MI6), by Charles Cumming

Brenna Miller: decoupage teacher in Morse Point, a small town in New England, in the Decoupage Murder mysteries by Lucy Lawrence (Jenn McKinlay)

Christopher Miller: FBI agent in Jersey City, New Jersey, by Chris Grabenstein

Hannah Miller (Granny Hanny), a mystery reader and quilt-making enthusiast, in Pennsylvania, in the Amish Country mysteries, by Barbara Workinger

Lydia Miller: 30-something fashion editor in New York City, by Eleanor Hyde

Nick Miller: sergeant detective at Central Division, in London, England, by Harry Patterson, later released as Jack Higgins

Robin Miller: lesbian romance novel and travel writer turned private investigator, in New York City, by Jaye Maiman

Kate Millholland: attorney in Chicago, Illinois, by Gini Hartzmark

Kinsey Millhone: private eye in fictional Santa Teresa, California, by Sue Grafton

Doug Milligan: police detective in the fictional beach community of Rocky Bluff, California, by F.M. Meredith (Marilyn Meredith)

Todd Mills: gay TV news reporter in Minneapolis, Minnesota, by R.D. Zimmerman

Dennis Milne: cynical detective sergeant in London, England, later on the run under an assumed name, by Simon Kernick

Milton “Milo” Milodragovitch: Korean war vet, former sheriff’s deputy, and alcohol and drug-challenged private eye in Montana, by James Crumley

Milos, Lucio, Ramon, and Neil meet for lunch and take turns to tell their stories, in the Lunch series by Derek Hansen

Max Mingus: ex-cop private investigator, in Miami, Florida, and Haiti, by Nick Stone

Matt Minogue: police sergeant detective in Dublin, Ireland, by John Brady

Tori Miracle: ex-New York City crime writer turned novelist, in Pennsylvania, in the Pennsylvania Dutch mysteries, by Valerie Malmont

Miro: stodgy middle-aged spy and assassin working for a US intelligence agency, “The Firm,” in Canada and Ireland, by Shaun Herron

Donna Miro and Lorna Doria in Massachusetts and New York City, by Heather Graham

Jerry Mitchell: former pilot turned submarine officer, dealing with the Russians, by Larry Bond

Meredith Mitchell: Foreign Service officer, and Alan Markby, a detective inspector, in the Cotswolds, England, by Ann Granger

Benny Mitchell: brash detective constable in Cloughton, Yorkshire, England, by Pauline Bell

Michelle (Mitch) Mitchell: 30-something single mother operating a dive shop in Huron Harbor, Michigan, by Doug Allyn

Scott Mitchell: private eye in England, by John Harvey

Inspector Steven Mitchell, of Scotland Yard, Claude Warrington-Reeve, barrister, and Dr. David Wintringham, in England, by Josephine Bell

Jack “The Jammer” Molloy: former Las Vegas casino concierge, now owner of the New York Hawks football team, by Mike Lupica

Tess Monaghan: private investigator in Baltimore, Maryland, by Laura Lippman

Greg Monarch: lawyer in La Graciosa, California, by Barry Siegel

Claude Monet: French impressionist painter, in early 1900s London, England, and Venice, Italy, by Jane Jakeman

Séraphin Monge: orphan returning to his hometown after WWI, in Provence, France, by Pierre Magnan

Richard Montgomery: detective inspector in Nottingham, England, by Stella Shepherd

Adrian Monk: obsessive-compulsive police detective, in San Francisco, California, in the Monk TV novelizations by Lee Goldberg

Dittany Henbit Monk: garden club member, and her husband Osbert Monk, an author, in Lobelia Falls, Ontario, Canada, in the Grub-and-Stakers series, by Alisa Craig (Charlotte MacLeod)

Richard Monk: lawyer in England, by Michael Underwood

William Monk: amnesiac police inspector in Victorian London, England, by Anne Perry

Carroll Monks: emergency-room doctor in San Francisco, California, by Neil McMahon

Adele Monsarrat: hospital nurse in Marin County, California, by Echo Heron

Louis Monroe: Don Quixote-style cop, in Atlantic City, New Jersey, by Bill Kent

Salvo Montalbano: police inspector in Sicily, Italy, by Andrea Camilleri

Lane Montana: finder of lost things, and Trey Fortier, a homicide detective, in Atlanta, Georgia, by Kay Hooper

Britt Montero: newspaper crime reporter in Miami, Florida, by Edna Buchanan

Luis Montez: attorney and former Chicano activist, in Denver, Colorado, by Manuel Ramos

Lucie Montgomery: operating her family’s winery in Virginia, in the Wine Country mysteries by Ellen Crosby

Suzie Montford: Woman Police Constable in London, England during WWII, by John Gardner

Freddie Montgomery: by John Banville

Grant Montgomery: 40-something retired mentalist and entertainer, in Pittsburgh, Pennsylvania, by Robert Aiello

Sarah Monteiro: international journalist from Portugal who investigates things Vatican, by Luís Miguel Rocha

Pat Montella: whose sleuthing is aided by her ability see and hear ghosts, on an estate near Fredericksburg, Virginia, by Elena Santangelo

Reuben Montoya and Rick Bentz, police detectives in New Orleans, Louisiana, by Lisa Jackson

Bridget Montrose: mother of small children in Palo Alto, California, by Lora Roberts

Claire Montrose: amateur sleuth in Oregon, by April Henry

Jean Montrose: physician in Scotland, by C.F. Roe

Stanley Moodrow: cop turned private investigator, in New York City, by Stephen Solomita

Scott Moody: an ex-mental patient cab-driving private investigator in the late 1970s, in Spokane, Washington, by Steve Oliver

Charlie Moon: Ute police officer in Grant Creek, Colorado, by James Doss

Louise Moon: mortuary technician at Charity’s (Charitable Hospital of St. Roche Without the Walls) mortuary, in London, England, by Anne Morgellyn

Phyllida Moon: private eye on television, in London, England, by Eileen Dewhurst

Stella Mooney: Detective Sergeant in London, England, by David Lawrence

Phil Moony: ex-fireman private investigator, in Chicago, Illinois, by Paul Engleman

Lily Moore: travel writer based in New York City, by Hilary Davidson

John Schuyler Moore: a turn-of-the-19th-century journalist in New York City, by Caleb Carr

Rebecca Moore: former reporter who runs a classic car restoration shop, in rural Maryland, by Judith Skillings

Sherry Moore: blind psychic who can relive a murder victim’s last moments, based in Philadelphia, Pennsylvania, by George D. Shuman

Skip Moore and James Lessor, best friends since grade school, in Florida, by Don Bruns

Toussaint Moore: black postal worker turned private detective, by Ed Lacy

Benita Morales: a financial analyst and boxer, and Dr. Saylor Oz, a sex therapist, in Brooklyn and Manhattan, New York City, by Allyson Roy

Mira Morales: bookstore owner and psychic, in Tango Key, Florida, by T.J. MacGregor

Cordelia Morgan: lesbian hit artist turned sleuth, in northern New Mexico and southern Colorado, by Bett Reece Johnson

David Morgan and Miranda Quin, a “virtual couple” of homicide detectives, in Toronto, Ontario, Canada, by John Moss

James Morgan: police chief in a suburb of Boston, Massachusetts, by Andrew Coburn

Lillian Morgan: retired math professor in a retirement community in Chapel Hill, North Carolina, by Alan Cook

Charlie Morell: Cuban-American private investigator turned lawyer in Los Angeles, California, by Alex Abella

Teddy Morelli: college history professor in Seattle Washington, by Linda French

Dexter Morgan: blood spatter technician for Miami Dade Police Department, and sociopathic serial killer in Florida, by Jeff Lindsay

Jake Morgan: disgraced Boston police officer turned blackjack dealer in Las Vegas, Nevada, by Rick Gadziola

Louis Morgon: Middle East policy expert dismissed from the CIA, taking refuge in France, by Peter Steiner

Rain Morgan: newspaper reporter in London, England, by Lesley Grant-Adamson

Taylor Morgan: medi-flight pilot in Alaska, by Megan Mallory Rust

William “Shark” Morgan and Dell Hassler, police detectives in Hilton Head, Beaufort County, South Carolina, by Vicky Hunnings

Paul Morganstein: lawyer, and his partner Ted Stevens, a hard-luck attorney, in Tallahassee, Florida, by Terry Lewis

Professor Moriarty: archenemy of Sherlock Holmes in London, England, by John Gardner

Professor Moriarty: the Napoleon of Crime and nemesis of Sherlock Holmes, in 1890s London, England, by Michael Kurland

Seth Mornay: a former Royal Marine, now a detective sergeant in the Criminal Investigation Division of the Grampian Police Force, in northeast Scotland, by M.G. Kincaid

John Morpugo: British counter-intelligence agent, by William Garner

Chantalene Morrell, daughter of a Gypsy mother and a redneck father, in Tetumka, Oklahoma, by M.K. Preston

Christine Morris: forensic profiler from Canada, in the Hebrides, Scotland, by Maureen Jennings

John Morris: Associated Press sportswriter focusing on golf and football, based in 1970s Atlanta, Georgia, by John Logue

Bev Morriss: lippy but loveable detective sergeant in Birmingham, England, by Maureen Carter

John Morrissey: chief inspector and family man, in Malminster, England, by Kay Mitchell

Alex Morrow: detective inspector in Glasgow, Scotland, by Denise Mina

Chief Inspector Morse: inspector in Oxford, England, by Colin Dexter

Charlie Mortdecai: dishonest art dealer who consorts with international crooks, and his sidekick Jock Strapp, in England, by Kyril Bonfiglioli

James Morton: upper-crust constable, and Joseph Bragg, a down-to-earth detective sergeant, in 1890s London, England, by Ray Harrison

Hoke Moseley: cop in Miami, Florida, by Charles Willeford

Zen Moses: cancer survivor and private investigator, in Santa Monica, California, by Elizabeth Cosin

Inspector Mosley, in Lancashire and Yorkshire border country in England, by John Greenwood (John Buxton Hilton)

Phil Moss and Colin Thane, Crime Squad officers in Glasgow, Scotland, by Bill Knox

Wiley Moss: graphic artist in Washington, DC, by Neal Barrett Jr.

Mr. Moto, a Japanese secret agent John P. Marquand

Angus Mott: a police sergeant, and Mike Yeadings, a police superintendent, in Thames Valley, England, by Clare Curzon

Colonel Mostyn: of Intelligence, by Mark Hebden

Charlie Muffin: experienced, rumpled, working-class British agent, by Brian Freemantle

Muirteach: scribe and former monk, in 1370s Scotland, by Susan McDuffie

Norah Mulcahaney: homicide detective in New York City, by Lillian O’Donnell

Kate Kincaid Mulcay: veteran newspaperwoman in Atlanta, Georgia, by Celestine Sibley

Muldoon and Tillman, policemen in 1880s New York City, by William Marshall

“Fang” Mulheisen:  police detective in Detroit, Michigan, by Jon A. Jackson

Liam Mulligan: street-smart investigative reporter in Providence, Rhode Island, by Bruce DeSilva

Tim Mulligan and Elsie Mae Hunt, professional archeologists and amateur sleuths, by Aaron Marc Stein

Al Mundy, in “It Takes a Thief” TV novelizations, by Gil Brewer

Dougal Munro: brigadier, and Jack Carter, a captain, in 1940s Ireland, by Jack Higgins

Kent Murdock: hard-drinking crime photographer, and frequent co-star, tough guy Jack Fenner, in Boston, Massachusetts, by George Harmon Coxe

William Murdoch: police detective in 1890s Toronto, Ontario, Canada, by Maureen Jennings

Matt Murdock: hard-boiled private investigator based in Newport Beach, California, by Robert J. Ray

Page Murdock: deputy US Marshal in 1880s American West (and Canada), by Loren D. Estleman

D.B. Murphy: ex-blacksmith private investigator in the 1920s-1930s, in the Georgian Bay port town of Owen Sound, Ontario, Canada, by Richard J. Thomas

Jack Murphy: ex-cop turned private investigator, in Los Angeles, California, by David J. Sherman

Jimmie Murphy: undercover police officer in London, England, by Colin Bateman

Maggie Murphy: recently widowed 24-year-old, whose dead husband’s ghost Horace hangs around, and patrolman Joe Driver, in Memphis, Tennessee, by Jackie Griffey

Molly Murphy: an Irish immigrant in early 20th-century who wants to be a private investigator, in New York City, by Rhys Bowen

Kate Murray: journalist, in Canada, by Emma Cole

Tom Musgrave, the Master of Defense, a sleuth in 1590s Elizabethan England, by Peter Tonkin

Hiawatha and Antigone Musing: siblings, professors, and part-time sleuths, in the Ivory Tower mysteries, by David D. Nolta