Stewart attended Mercersburg Academy prep school, graduating in 1929. He was active in a variety of activities. He played on the football and track teams, was art editor of the KARUX yearbook, and a member of the choir club, glee club, and John Marshall Literary Society. During his first summer break, Stewart returned to his hometown to work as a brick loader for a local construction company and on highway and road construction jobs where he painted lines on the roads. Over the following two summers, he took a job as an assistant with a professional magician. He made his first appearance onstage at Mercersburg, as Buquet in the play The Wolves.