After graduating Principia College in 1953, Duvall decided to follow a path similar to his father's and enlisted in the U.S. Army. During his time in the Army, Duval leveraged his passion and took up acting. While stationed at Camp Gordon, he acted in plays such as "Room Service." Later, due to his having served during the years of the Korean War, the media would often misunderstand his participation during the fighting. He left the military after two years of service at the rank of Private First Class.