While attending the University of the Pacific, McGavin took a side job building scenery for a local theater group. He subsequently dropped out of college, and found work as a painter at Columbia Pictures movie studios in 1945. When an opening became available for a bit part in "A Song to Remember", McGavin applied and won his first movie role. Shortly afterwards, he moved to New York City and studied at the Neighborhood Playhouse and the Actors Studio under teacher Sanford Meisner. In 1949, he joined the cast of a touring production of "Death of a Salesman", playing Happy Lohman. He began appearing on Broadway in 1954, in productions of "My 3 Angels" and "The Rainmaker" (where he created the title role) opposite Geraldine Page. While in New York, McGavin also appeared on several live theater programs that aired on television, such as "Kraft Television Theatre" and "The U.S. Steel Hour".