Believed to have committed 17 killings as a hired gunman throughout the West,[2] Horn was convicted in 1902 of the murder of 14-year-old Willie Nickell near Iron Mountain, Wyoming.

[5] At 16, Horn headed to the American Southwest, where he was hired by the U.S. Cavalry as a civilian scout, packer, and interpreter under Al Sieber during the Apache Wars.

In one instance, as the army was crossing Cibecue Creek in Arizona, they were ambushed by Apache warriors positioned on high ground.

By November 1885, Tom Horn earned the position of chief of scouts under Captain Emmet Crawford in Fort Bowie.

[8][9] During one operation, Horn's camp was mistakenly attacked by a Mexican militia, and he was wounded in the arm during the shootout, which also resulted in Crawford's death.

However, it was short-lived, as cattle thieves stormed his ranch one night and stole all his stock, resulting in a tremendous loss and bankruptcy for Horn.

This incident marked the start of Horn's hatred and disdain for thieves, which led to his entering the profession of range detective.

If he thought a man was guilty of stealing cattle and had been fairly warned, Horn said that he would shoot the thief and would not feel "one shred of remorse".

[17] He claimed that throughout the war he was the "mediator" of the conflict, serving as a deputy sheriff under three famous Arizona lawmen: Buckey O'Neill, Perry Owens, and Glenn Reynolds.

As a deputy sheriff, Horn drew the attention of the Pinkerton National Detective Agency owing to his tracking abilities.

Hired by the agency in late 1889 or early 1890, he handled investigations in the Rocky Mountains of Colorado, Wyoming, and other western states, working out of the Denver office.

Shores, captured two men who had robbed the Denver and Rio Grande Western Railroad on August 31, 1890, between Cotopaxi and Texas Creek in Fremont County, Colorado.

[1] In 1896, Horn offered his service in a letter to the marshal of Tucson, Arizona, in getting rid of William Christian's rustler gang.

On the day of the murder, an armed Horn allegedly arrived at Rash's cabin just as he finished eating and shot him at point-blank range.

Rash was supposed to be married to a nearby rancher, Ann Bassett, and the woman accused "Hicks" of being the murderer.Around the same time, Horn also suspected another cowboy named Isom Dart of rustling.

[26] Dart was one of Rash's fellow cowboys but was believed to have previously worked as a rustler named Ned Huddleston and to have been a former member of the late "Tip Gault" gang.

Prior to the assassination, Horn had instructed a rancher named Robert Hudler to ready a horse miles from the murder scene for his getaway.

[28] The next day, two spent .30-30 Winchester shell casings were found at the base of a tree where the murderer is believed to have lain in wait.

During the Wilcox train robbery investigation, Horn obtained information from Bill Speck that revealed which of the outlaws, George Curry or Harvey Logan, had killed Sheriff Josiah Hazen during their escape.

Horn personally witnessed the bravery of the famous Rough Riders and colored regiments, the Ninth and Tenth Cavalries, during their assault on San Juan Hill, as well as the humiliating rout of American soldiers under Brigadier General Hamilton S. Hawkins.

Shortly after his return, Horn began working in 1901 for wealthy cattle baron John C. Coble, who belonged to the Wyoming Stock Men's Association.

[32] While working again near Iron Mountain, Wyoming, on July 15, 1901, Horn visited the Jim and Dora Miller family, who were cattle ranchers.

Marshal Joe LeFors came to Iron Mountain and arrested Jim Miller and his sons Victor and Gus on suspicion of shooting Kels Nickell.

Horn allegedly confessed to killing young Willie with his rifle from 300 yards (270 m), which he boasted of as the "best shot that [he] ever made and the dirtiest trick that [he] had ever done".

He gathered a team for the defense headed by former Judge John W. Lacey, and which included attorneys T. F. Burke, Roderick N. Matson, Edward T. Clark, and T. Blake Kennedy.

He wrote that 100 members of the Wyoming Stock Growers Association paid $1000 each[dubious – discuss] toward the defense, but wanted a minimal effort.

The prosecution introduced testimony by at least two witnesses, including Lefors, as well as circumstantial evidence; these elements only placed Horn in the general vicinity of the crime scene.

[15] Glendolene Kimmell had testified during the coroner's inquest, saying she thought both the Miller and Nickell families responsible for maintaining the feud, but she was never called as a defense witness.

Convinced of Horn's innocence, Glendolene Kimmell sent an affidavit to Governor Fenimore Chatterton with testimony reportedly saying that Victor Miller was guilty of Nickell's murder.

[33] After his death, many considered Horn to have been wrongly executed for a murder solely based on a purported confession given when drunk, thus of dubious admissibility in court.