Williams began pursuing an acting career after spending two years as a paratrooper in C Company, 506th Infantry, of the 101st Airborne Division. He first appeared on Broadway in "The Long Dream" (1960). Continuing his work on stage, he appeared in "Walk in Darkness" (1963), "Sarah and the Sax" (1964), "Doubletalk" (1964), and "King John". His breakout theatrical role was in William Hanley's "Slow Dance on the Killing Ground", for which he received a Tony Award nomination. The "New York Times" drama critic Howard Taubman wrote of his performance, "Mr. Williams glides like a dancer, giving his long, fraudulently airy speeches the inner rhythms of fear and showing the nakedness of terror when he ceases to pretend." He also served as artist-in-residence at Brandeis University in 1966.