"[5] On Broadway, Armstrong portrayed Dr. Baugh and Big Daddy in Cat on a Hot Tin Roof (1955), Sheriff Talbott in Orpheus Descending (1957), and Captain Keller in The Miracle Worker (1959).

He guest-starred in virtually every television Western series produced in the 1950s and 1960s, including Have Gun - Will Travel playing Sheriff Jaffey in S1 E28 "Killer's Widow" which aired 3/21/1958, 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 (S7E10 as hard nosed Union soldier Capt.

Armstrong also appeared on Alfred Hitchcock Presents, The Everglades, The Andy Griffith Show, The Fugitive, Daniel Boone, T.H.E.

This character archetype appeared in Ride the High Country (1962), Major Dundee (1965), and perhaps most memorably in Pat Garrett and Billy the Kid (1973).

Some of his more memorable roles outside of Peckinpah's films include a sympathetic rancher in El Dorado (1966), Cap'n Dan in The Great White Hope (1970), outlaw Clell Miller in The Great Northfield Minnesota Raid (1972), a bumbling outlaw in My Name is Nobody (1973), a secret Satanic cultist sheriff in Race with the Devil (1975), The Car (1977), as well as Children of the Corn (1984), Red Headed Stranger (1986) with Willie Nelson, and as General Phillips in Predator (1987).

He appeared in several of Warren Beatty's films, including Heaven Can Wait (1978), Reds (1981), and as the character Pruneface along with Al Pachino, Dustin Hoffman, Ed O'Ross, James Tolkan, Mandy Patinkin,Catherine O'Hara, William Forsythe and Madonna in Dick Tracy (1990).

He semi-retired from films and television in the late 1990s, but he continued to be active in off-Broadway theater in New York and Los Angeles, until finally retiring from acting in 2005 because of near-blindness due to cataracts.