Ustinov wrote seven plays that were produced on Broadway, and he performed in or directed five of them. The most successful of these productions, the Cold War satire Romanoff and Juliet, ran for 389 performances in 1957 and 1958 and was filmed under Ustinov's direction in 1961 (George S. Kaufman had staged the Broadway production). Ustinov received Tony nominations for both writing and acting in the play.
His final Broadway appearance was in his play Beethoven's Tenth, which ran briefly at the Nederlander Theater in 1984. Ustinov portrayed "Ludwig" and George Rose appeared as "a music critic" in the comedy; the cast also included Gwyllum Evans, Neil Flanagan, Gina Friedlander, Mary Jay, Anderson Matthews, Leslie O'Hara, and Adam Redfield. Ustinov's other Broadway shows were The Love of Four Colonels (1953), Photo Finish (1963), The Unknown Soldier and His Wife (1967), Halfway Up the Tree (also 1967), and Who's Who in Hell? (1974).
In addition to playwriting, acting, and directing, Ustinov was an accomplished novelist and cartoonist, and he served as a goodwill ambassador for UNICEF for many years. He was appointed a Commander of the British Empire in 1975.