Born on New York City's East Side, Billy as a child migrated with his parents to Kansas; his father died there, and the mother and her two boys moved to Colorado, where she remarried. The family moved to New Mexico, and, in his early teens, Billy fell into a career of thievery and lawlessness, wandering throughout the Southwest and northern Mexico, often with gangs. In December 1880 he was captured by Sheriff Patrick Floyd Garrett (Garrett, Pat) and stood trial for murder in Mesilla, N.M., in April 1881; he was found guilty and sentenced to hang. He escaped jail on April 30, however, killing two deputies, and remained at large until tracked down and ambushed by Garrett, who shot him dead on the evening of July 14 at the ranch home of Pete Maxwell. Billy the Kid's grave is in Fort Sumner, N.M.

As a child, Billy the Kid went under the name of Henry McCarty. Scholarly opinion is divided over whether that or William H. Bonney, Jr. (the name he used later, as in the trial), was his true name. Another hypothesis is that Billy the Kid was in fact Ollie L. “Brushy Bill” Roberts, who escaped, lived in Mexico and the U.S. Southwest, rode in Wild West shows, and died in 1950 in Hico, Texas.