His first play on Broadway – "Command Decision" – in which Whitmore played the part of Tech Sergeant Harold Evans, was the smash hit of 1947, and Whitmore won the Tony Award for "Best Newcomer of the Season." Whitmore continued to be active in the theatre for all of his career, performing on Broadway, at Ford's Theatre in Washington DC, and on tour. He later won the title "King of the One Man Show" after appearing in the solo vehicles "Will Rogers' USA" (1970) (repeating the role for TV in 1972); as Harry Truman in "Give 'em Hell, Harry!" (1975) (repeating the role in the film version, for which he was nominated for an Oscar); and as Theodore Roosevelt in "Bully" (1977), although the latter production did not repeat the success of the first two.