Jacob B. Matthews (18??-1904) - A gunman and lawman, he served in the Tennessee Cavalry in the Civil War and was later a deputy sheriff under Sheriff Brady during the Lincoln County War in New Mexico.

D. Boone May (1852-1910) - A gunman, stage guard, and U.S. Deputy Marshal in Deadwood, South Dakota and Cheyenne, Wyoming, he killed several stage robbers in the 1870s.

Thomas P. McCall - The first sheriff of Medina County, Texas in 1858. Later he served as a deputy sheriff in Bexar County, before becoming sheriff and serving for ten years.

William McCauley - Served as a Texas Ranger in the 1890s and in 1897 helped break up a gang terrorizing San Saba, Texas.

Robert McClure - Served as a Texas Ranger in the 1890s and in 1897 helped break up a gang terrorizing San Saba, Texas.

Mike McCluskie, aka: Arthur Delaney, Art Donovan (18??-1871) - A little known gambler and occasional lawman , McCluskie instigated the Hide Park Gunfight on August 19th, 1871, in Newton, Kansas when he killed Billy Bailey a week earlier. When the smoke cleared he was severely wounded, shot by Hugh Anderson. He died six hours later.

Myron McCord - Appointed as a U.S. Marshal in Arizona Territory on June 6, 1901.

O.W. McCuiston - While sheriff of Raton, New Mexico in 1894, he became embroiled in a railway strike.

William Jesse McDonald (1852-1918) - Deputy sheriff in Mineola, Texas; U.S. Deputy Marshal in Indian Territory; captain in the Texas Rangers. Died of pneumonia in 1918.

James "Jim" McIntire (or McIntyre) (1846-1902) - A Texas gunman and lawman from 1860, he was a Texas Ranger, city marshal of Las Vegas, New Mexico, deputy sheriff, cowboy, hunter, gambler, and outlaw with a $1,000 reward on his head for the deaths of two men near Silver City, New Mexico.

T.W. McIntosh - Served as a deputy marshal in Prescott, Arizona.

Joe McKidrict - Served as a Texas Ranger, he later shot and killed U.S. Deputy Marshal, Bass Outlaw.

Joseph "Joe" W. McKidrict (1871-1894)  - Twenty-three year-old Texas Ranger, McKidrict was shot and killed by U.S. Deputy Marshal, Bass Outlaw on April 5, 1894 in El Paso, Texas. 

Joseph McKinney (18??-1850) - Sheriff of Sacramento, California in 1850, he was shot and killed by squatters.

Thomas L. "Tip" McKinney - A New Mexico gunman who was also a lawman, accompanied Pat Garrett when he killed  Billy the Kid on July 14, 1881.

Sherman W. McMasters (1853-??) - McMasters was an outlaw turned lawman who would become one of the six men involved in the Earp Vendetta Ride. It is unknown when or how he died.

Francis Marion McMahon (18??-1940) - Served in Company D of the Texas Rangers under John Reynolds Hughes in September, 1893 and arrested Bass Outlaw on April 4, 1894. He later served as a U.S. Deputy Marshal.

Sam A. McMurry - Served for several years in the 1880-90s as captain of Company B of the Texas Rangers.

J.S. McNeel - Served in Company E as a Texas Ranger in 1892.

P.J. McNeel - Served in Company E as a Texas Ranger in 1892.

Leander H. McNelly (1844–1877) - A Texas lawman, McNelly was commissioned as captain of the state police on July 1, 1870, and commanded the "Special Ranger Force" in 1874 which patrolled the Mexico border. He died on September 4, 1877.

William McNew - Deputy marshal of Dona Ana County, New Mexico.

John McPherson (18??-1879) - Police Chief in Las Vegas, New Mexico. Killed in a gunfight.

William Kidder Meade (1851-1918) - Appointed as a U.S. Deputy Marshal for Arizona Territory on July 8, 1885.

Mike Meagher (1844–1881) - Wichita, Kansas Marshal, U.S. Deputy Marshal in Indian Territory, and Caldwell, Kansas Mayor.

Joseph Lafayette Meek (1810-1875) - After heading west from Virginia, Meek first spent his life as a mountain man before becoming a sheriff in the Willamette Valley of Oregon. Later won a seat in the legislature and appointed Oregon territory's first federal marshal.

Hod Miles - An outlaw turned deputy sheriff, who in January 1888, killed Jake Gibson, who was allegedly his fifth victim.

F.M. Miller - Female U.S. Deputy Marshal. Commissioned out of the federal court at Paris, Texas, she was the only female deputy working in the Indian Territory in 1891. Known to always wear a cowboy hat and adorned with a gun belt filled with cartridges and a Colt pistol, she had a reputation as a fearless and efficient officer who locked numerous outlaws. The attractive brunette, who was described as having charming manners, also carried a Winchester rifle strapped to her saddle and was an expert shot and superb horsewoman.

James B. Miller, aka: Killin' Jim, Killer Miller, Jim the Killer, Deacon Miller (1866-1909) - Killing his first man when he just 18, Miller became a hired gun in the 1880s, a "career" he would continue for the next two decades. He served as an occasional lawman in Pecos, Texas.  He killed Sheriff Bud Frazer and was suspected of forty other killings including Pat Garrett and A.A. Bobbitt. On April 19, 1909, he was hanged by vigilantes in Ada, Oklahoma.

John Miller - A U.S. Deputy Marshal in Tucson, Arizona during the 1870s.

Zeke Miller - U.S. Deputy Marshal in Indian Territory.

Alexander Hamilton Mills (1837-1882) - Known as a "crack-shot," Mills was the sheriff Lincoln County, New Mexico during the Horrell War. He later got into trouble on embezzling charge and manslaughter charges in 1875. He was killed by Mexicans in 1882.

Jeff Davis Milton (1861-1947) - A fearless lawman , Milton's career spanned more than half a century as he as police chief of EI Paso, Texas; a Texas Ranger, and a U.S. Deputy Marshal in New Mexico and Arizona.

David Montgomery - Serving as Chief Deputy Marshal of Lincoln County, New Mexico in 1878.

John "Happy Jack" Morco - A gunman and lawman in Ellsworth, Kansas. He he killed in a gunfight there with Policeman J. C. Brown.

Joe Morgan - A gunman and lawman, he fought Deputy Sheriff Ben Williams in Las Cruces, New Mexico on September 15, 1895, with Albert Fall. He was later a deputy sheriff and Oliver Lee supporter.

Jeff B. Moore - A cowboy, mercenary and lawmen, he fought Indians in Argentina before he served several terms as sheriff of Crockett County, Texas.

John Moore - A Texas Ranger in 1896.

Joe Morgan - A gunman and lawman, he was in a gunfight with Ben Williams in Las Cruces, New Mexico on September 15, 1895. Both he and Williams were wounded.

W.T. "Brack" Morris (18??-1901) - Served in Company D of the Texas Ranger in 1882 and was sheriff of Kames County, Texas, when he was killed on June 13, 1901.

Alexander L. Morrison, Jr. - Served as chief deputy to his father in Santa Fe County, New Mexico.

Alexander L. Morrison, Sr. - Appointed marshal of Santa Fe, New Mexico in March, 1882.

Harry N. Morse (1835–1912); Sheriff, Alameda County, California.

George Nelson Moses (1844-1911) - A U.S. Deputy Marshal in Arizona and New Mexico, and later the first sheriff of Barton County, Kansas. He later became the mayor of Great Bend, Kansas.

Burton C. Mossman (1867–1956) - Managed the Hashknife Outfit of Arizona to try to stop the rampant cattle rustling before becoming the first captain of the Arizona Rangers in 1901.

George W. Mowbray - Worked as a posseman under Heck Thomas, Bill Tilghman, and Chris Madsen in the early 1890s and was later appointed as a U.S. Deputy Marshal himself. Over the years, he fought the Cook gang, the Doolin Gang, the Buck Gang, and Cherokee Bill's gang.