He moved to Douglas, Arizona around 1914, where he worked as a cowboy. In 1916, he joined the U.S. Army, and served under General Jack Pershing in pursuit of Pancho Villa in Mexico. He was later assigned to Fort Bliss, on the Texas–New Mexico state line. In the 1920 census for El Paso, his profession is listed as blacksmith. While at Fort Bliss, he was injured when kicked in the head by a horse, causing him severe headaches for the rest of his life. He was discharged from the army in 1920.
He worked for two years for the El Paso Police. He was partnered with officer Juan Escontrias, and the two were involved in two shootouts during that time with smugglers, resulting in four suspects being killed, and Threepersons being shot and wounded in the chest during one incident in 1921. On June 10, 1922, Threepersons was appointed as a Federal Probation Agent for El Paso, but he kept the job only a few months, resigning to manage the "Cudahy Ranch" in Durango, Mexico. During his brief employment for the ranch he killed two rustlers during a shootout. He was arrested by Mexican authorities for the shooting, but escaped and returned to the United States.
In July 1923, Threepersons accepted a position as a Mounted Inspector for the U.S. Customs Service. That same year he was run-over by a bootlegger's vehicle during an arrest, which resulted in injuries, none serious. From 1925 he worked for both the El Paso County Sheriffs Office and the El Paso Police Department. Locally, Threepersons was well known for his exploits, and starting in 1925, the S.D. Myres Saddle Co., of El Paso, began advertising the "Tom Threepersons-style holsters". The holster, which included a cutaway top exposing the pistol hammer and trigger guard, became popular and was copied by several other manufacturers.
Threepersons was offered a job in Hollywood, California, in the film industry, reputedly at a salary of $700 per month, which he declined.