Mosley was born in Los Angeles and made his debut in 1971 episodes of Longstreet and Cannon. In 1973, he worked with Selleck for the first time in Stephanie Rothman's Terminal Island. He had several minor roles in movies and television in the early 1970s, even working with John Wayne in McQ. In 1976, he scored his most prominent film role, playing legendary folk singer Huddie "Lead Belly" Ledbetter in Gordon Parks' Leadbelly biopic.
Throughout the rest of the 1970s, Mosley continued appearing in hit movies and shows. He appeared in The Rockford Files, Starsky and Hutch, and Roots: The Next Generation. He played boxer Sonny Liston in The Greatest, in which Muhammad Ali played himself. Mosley also starred in Burt Reynolds' football comedy Semi-Tough.