In Cat, Mr. Armstrong played the family doctor who keeps from Big DDaddy the information that he is dying. In Orpheus Descending, he was the corrupt town sheriff.
Other Broadway credits included The Miracle Worker and The Long Dream.
Mr. Armstong was far busier in the fields of film and television, where he appeared in many westerns. A friend of filmmaker Sam Peckinpah, he was featured in a few of the director's movies. He was also a favorite of director Warren Beatty. His movie credits include "The Fugitive Kind" (the film version of Orpheus Descending), "Ride the High Country," "Major Dundee," "Pat Garret and Billy the Kid," "El Dorado," "The Great White Hope," "Reds" and "Heaven Can Wait."
On television, he acted in the series "Have Gun—Will Travel," "The Westerner," "Bonanza," "Maverick," "Gunsmoke," "Rawhide" and "Wagon Train," as well as "The Twilight Zone" and "Alfred Hitchock Presents."
He is survived by his four children, from his first marriage, to Ann Neale.