The supporting actor, who began in films in 1947, was known for playing ethnic types (usually Greek) in films such as Kiss Me Deadly and the Humphrey Bogart film Sirocco. Dennis, who spoke Greek fluently, appeared in a number of television programs in the 1960s and 1970s including playing the parts of Orderly Nick Kanavaras on the medical drama Ben Casey and Uncle Constantine on the detective show Kojak.

Nick Dennis also played the role of Pablo Gonzales in Tennessee Williams' play, A Streetcar Named Desire, as well as its subsequent film version in 1951.