A script supervisor for Robert Wise, Flynn was an assistant director by the mid '60s, and helmed his first film in 1968, the gay-themed drama The Sergeant. He has since specialized in actioners, including the crime film The Outfit, which he also wrote; Rolling Thunder, co-scripted by Paul Schrader; the prison film Lock Up with Sylvester Stallone; Out For Justice with Steven Seagal; and Nails with Dennis Hopper.