The German-born Sommer made his first stage appearance as a child performer with the Carolina Playmakers in 1943. After a stint in the US Army, he trained in earnest at the American Shakespeare Festival in Stratford, CT, where he gradually worked his way up from small parts to major roles. Sommer also appeared in a number of productions at the Seattle Repertory Theatre before making his Broadway debut in a production of "Othello" in 1970. He has subsequently appeared in "The Shadow Box" (1977), "Spokesong" (1979) and David Hare's "Racing Demon" (1995).
On TV, Sommer acted as host and narrator of the PBS miniseries version of "The Scarlet Letter" (1979) and went on to portray a number of real-life individuals, notably President Gerald Ford opposite Gena Rowlands in "The Betty Ford Story" (ABC, 1987), Franklin D. Roosevelt in "The Kennedys of Massachusetts" (ABC, 1990) and Joseph P. Kennedy in "A Woman Named Jackie" (NBC, 1991). He has also been featured in two short-lived series, "Hothouse" (ABC, 1988), as the patriarch of a family of doctors, and "Under Cover" (ABC, 1991), which followed the exploits of an espionage team.