"Whitmore often said he found acting in films and television boring because of the long waits between scenes; his passion was for the theater, and he continued to act on stage throughout his long career." Whitmore first ventured into acting at Yale University – severe knee injuries sidelined him from football, so he turned to the Yale Dramatic Society. After serving in the Marines he toured the South Pacific in a USO tour, then returned to America, where he studied acting for six months at the American Theatre Wing in New York and the Actors Studio. Afterward, he was hired by a summer stock company in Peterborough, New Hampshire – The Peterborough Players. In 1947 he was selected to appear in a production of "All My Sons", representing the U.S. at the World Youth Festival in Prague.