He is best known for playing Pontius Pilate on the original 1970 recording of Jesus Christ Superstar as well as the subsequent 1971 stage musical and 1973 film adaptation.

They lived together for a year, during which time he helped her develop the nightclub act that began her successful career as a singer and actress.

Dennen also played a variety of small parts on American television shows including: Batman, Wonder Woman (where he portrayed a clone of Adolf Hitler), Galtar and the Golden Lance, Tales from the Darkside and Justice League: Throne of Atlantis, and films, such as Madhouse (1974), Brannigan (1975), The Kentucky Fried Movie (1977), The Shining (1980), Ragtime (1981), Trading Places (1983), Superman III (1983), Twin Sitters (1994) and Titanic (1997).

The show, written with writer Linda Bergman and composer Jeff Rizzo, is licensed by Samuel French.

His autobiographical book My Life with Barbra: A Love Story (1997) deals with their relationship and with his gradual realization that he was gay.