The Authentic Life of Billy, the Kid, The Noted Desperado of the Southwest is a biography and partly first-hand account written by Pat Garrett, sheriff of Lincoln County, New Mexico, in collaboration with a ghostwriter, Marshall Ashmun "Ash" Upson.

As the author wrote in his introduction to this biography, "I am incited to this labor, in a measure, by an impulse to correct the thousand false statements which have appeared in the public newspapers and in yellow-covered, cheap novels.

Many people had begun to gossip about the unfairness of Garrett's final encounter with the Kid, so his first reason, which was to clear his name, was decidedly his main purpose.

[6] Still smarting from local outrage in New Mexico over his shooting of the Kid,[7] Garrett wanted to present his side of the story and hoped to turn a profit, as well, on the American public's fascination with the outlaw.

[8] It was credited to Garrett, but the first 15 chapters were a concoction of factual material mixed with fabrications, written by Roswell's postmaster, Ash Upson, an itinerant journalist.