In 1970, Williamson made his feature film bow in Robert Altman's M*A*S*H*. He went on to appear in another 22 films in the 1970s, many of them blaxploitation flicks like "The Legend of Nigger Charley" (1972), "Hell Up in Harlem", "Black Caesar" (both 1973) and "Bucktown" (1975). Starting with "Boss Nigger" (1974), Williamson began writing, directing and/or producing the films in which he starred. Most were amiably low-budget action and crime films: "No Way Back", "Joshua" and "Mean Johnny Barrows" (1976); "One Down, Two to Go" (1982); "The Big Score" (1983); "Foxtrap" (1985); "Silent Hunter" (1995); and "From Dusk Till Dawn" (1996), as a biker. His handsome, mustachioed looks might have channeled him into big-budget leading man status, but Williamson found actioners more to his taste. He has also starred in a number of Italian-made films (including "I Nuovi Barbari" 1982 and "Vivre pour Survivre" 1984).
Williamson has appeared on television from time to time, announcing football games on ABC (1974) and acting in the mini-series "Arthur Hailey's Wheels" (NBC, 1978) and in the short-lived sitcom "Half-Nelson" (NBC, 1985) co-starring with Joe Pesci.
Williamson produced and co-starred with fellow 1970s Blaxploitation stars Richard Roundtree, Pam Grier, Jim Brown and Ron O'Neal in "Original Gangstas" (1996), an actioner about former gang members who reteam to rid their neighborhood of drug dealers.