Armstrong's first film appearance was in the 1954 film Garden of Eden; however, it was television where he first earned a name for himself. He guest-starred in virtually every television western series produced in the 1950s and 1960s, including Have Gun - Will Travel, The Californians, Jefferson Drum, The Tall Man, Riverboat, The Rifleman, Zane Grey Theater, Wanted: Dead or Alive, The Westerner, The Big Valley, Bonanza, Maverick (as Louise Fletcher's character's father in the episode which drew the series' largest single viewership, "The Saga of Waco Williams"), Gunsmoke, Rawhide, and Wagon Train.