Dennen moved to London, England for fifteen years and in 1968, landed the starring role of the master of ceremonies in the London version of Cabaret. In 1970, he played Pontius Pilate on the album of Jesus Christ Superstar, and he played the same role in the Broadway production. In the same year, he played Mendel in the Norman Jewison film Fiddler on the Roof. He suggested to Jewison that he direct a film version of Superstar. Jewison did so, and Dennen played Pilate again. In 1982, Dennen played the role of Buddy in the TV adaptation of Beau Geste.
For Disney, he voiced Morloon the Wizard and Vacation Von Honk in DuckTales, Nester in Adventures of the Gummi Bears, Ramsis Dendup in Star Wars: The Clone Wars, and a Chamberlain in The Dark Crystal.