On the small screen, Armstrong made his series debut as the original marshal on "Rifleman" (1958), and in the same year was a regular player on the Disney produced western series "Texas John Slaughter". Armstrong has appeared in a number of TV-movies including the Westerns "Hec Ramsey" (1972) and "The Shadow Riders" (1982) as well as the political thriller "Kingston: The Power Play" (1976), and the sci-fi adventure "The Time Machine" (1978). He made his miniseries debut as General "Moose" Fitzgerald in the epic "War and Remembrance" (1988).
Armstrong has perhaps found his steadiest supporter in director Steve Carver, veteran maker of masculine genre fare who began his career under Roger Corman. Since appearing in Carver's "Fast Charlie - The Moonbeam Rider" (1978) about a motorcycle race, Armstrong has been seen in the Carver features "Steel" (1980); "Lone Wolf McQuade" (1982); "Jocks" (1986); "Bulletproof" (1988); and "Dead Center" (1994).