Formerly a schoolteacher, Dunnock made her stage debut in 1932 and won acclaim on Broadway in 1940 as a Welsh teacher in Emlyn Williams' autobiographical drama "The Corn Is Green", a role she reprised in her film debut in 1945. Although she is memorable in the brief role as the wheelchair bound victim whom Richard Widmark pushes down the stairs in "Kiss of Death" (1948), Dunnock gave her finest performances as seemingly genteel spinster types who display surprising inner strength and sympathy.
Dunnock studied acting with Actors Studio founders Lee Strasberg, Robert Lewis and Elia Kazan and after directing her in "Death of a Saleman", Kazan repeatedly cast her as a figure of quiet moral authority in such films as "Viva Zapata!" (1952) and as Aunt Rose Comfort in Tennessee Williams' "Baby Doll" (1956) for which she received her second supporting actress Oscar nomination. Evidently a favorite actress of Williams as well as Kazan, she continued her association with the playwright on Broadway, creating the role of Big Mama in "Cat on a Hot Tin Roof" (1955), appearing in "The Milk Train Doesn't Stop Here Anymore" (1963) and starring in a 1966 regional revival of "The Glass Menagerie". She was also featured as Aunt Nonnie in Richard Brooks' 1962 film adaptation of "Sweet Bird of Youth".
Although she didn't begin acting professionally until she was in her 30s, Dunnock maintained an active career as a superb, understated character actress on stage, screen and TV. Her other notable films include Alfred Hitchcock's "The Trouble With Harry" (1955), "Love Me Tender" (1956), "Peyton Place" (1957), "Butterfield 8" (1960) and John Ford's last feature "Seven Women" (1966).