He is best known for obtaining the confession that led to the conviction of gunman Tom Horn in 1903 for the alleged murder of 14-year-old sheepherder Willie Nickell.
He played a minor role in the 1887 recovery of a large herd of cattle rustled by the Hole in the Wall Gang.
In 1899, Lefors took part in a posse to capture those responsible for what would become known as the "Wilcox Train Robbery", committed by the Hole in the Wall Gang led by outlaw Butch Cassidy.
[3] Thieves again robbed another Union Pacific train on August 29, 1900, near Tipton, Wyoming about fifty miles west of Rawlins, and this time Lefors led the posse.
Horn generated productive information which he later passed to Charlie Siringo via the agency and which he had obtained from explosives expert Bill Speck.
Through that information, the investigators were able to establish that Sheriff Josiah Hazen had been killed during a chase by the train robbery suspects.
[citation needed] Following the hanging of Horn, Lefors took the job offered in Helena, working there for many months, without success in countering the gang.