As a lawyer in Mesilla, Fountain's most famous client was Billy the Kid. Fountain lost the 1881 case, and Billy the Kid was convicted of murder despite the evidence, though he escaped from jail. In 1888, Fountain was elected to the New Mexico Territorial Legislature, defeating his enemy Albert Bacon Fall. At the time of Fountain's disappearance, he was investigating and prosecuting suspected cattle rustlers, specifically Oliver M. Lee, and he again found himself at odds with Fall, who was Lee's attorney.