Mosley was born in Los Angeles, California. He grew up in the Imperial Courts project with his mother Eloise Harris in Watts, one of the most dangerous parts of the inner city. In 1974, he founded the Watts Repertory Company.
Mosley's most prominent film role to date was his 1976 starring turn as the title character in Leadbelly, directed by Gordon Parks. He has guest starred on shows such as Night Court, Starsky and Hutch, Kojak, The Rockford Files, Baretta, and Sanford and Son; he also had a role in Roots: The Next Generation. He also made a memorable appearance in the 1973 film The Mack, as the militant brother of the main character Goldie, and played officer Roy Cole alongside Kurt Russell and Ray Liotta in Unlawful Entry (1992).
Most recently he appeared in season five of Las Vegas as the billionaire friend of Montecito owner AJ Cooper (Tom Selleck). There is a vague homage to his Magnum P.I. days as his character 'Roger' is worth more than $2bn and owns a fleet of jets having started with a single helicopter in Hawaii.