Thomas Horn was born near Memphis on 21st November, 1860. At the age of 14 he ran away from home and went to Santa Fe where he worked as a stage driver. During this period he learnt Spanish and became known for his skills with a gun.

In 1882 Horn joined the U.S. Army and took part in the Tupper-Rafferty-Enmedio fight in Sonora. The following year Horn was recruited by Al Sieber as a scout on the Sierra Madre expedition led by Major General George Crook. In 1885 Horn was appointed chief of scouts for the pursuit of Geronimo in the Sierra Madre and was involved in his capture in September 1886.

With the defeat of Geronimo, Horn was no longer required as a scout and was dismissed from the army. He moved to Aravaipa, Arizona, where he worked in the mining industry. Later he worked as a cowboy and one a steer-roping contest at Globe, Arizona, on 4th July, 1888. Horn also won a similar contest in Phoenix.

In 1890 Horn joined the Pinkerton Detective Agency at Denver. Four years later he became a detective with the Swan Land and Cattle Company. During this period it appears he became an hired assassin. He later admitted to a friend that he murdered William Lewis (August, 1895) and Fred Powell (September, 1895) for $1,200. Horn also approached William K. Meade, the marshal of Tucson, and offered to kill members of the Christian gang. It is not known whether Meade commissioned Horn to do this but Jack Christian and Robert Christian were both murdered in 1897.

Horn was recruited by the US Army to fight in the Spanish American War. When he was discharged in September 1898 he returned to working as a hired assassin and killed Madison Rash (8th July, 1900) and Isom Dart (3rd October, 1900) in Colorado. He later told a friend that he had been hired to kill these two men because they were rustlers.

On 19th July, 1901, a 13 year old boy, William Nickell, was killed by a sniper in the Iron Mountain region. The murder took place in bad light and Marshal John LeFors became convinced that he had been mistaken for his father, Kels Nickell. LeFors interviewed Horn about the death of Nickell. He admitted he had been employed to stop rustlers stealing cattle in Montana and that he had killed Nickell by mistake.

During his trial Horn claimed he had been drunk when he was interviewed by LeFors and had lied about the killing of William Nickel. The jury decided that Tom Horn had killed Nickel and he was hanged on 20th November, 1903.