O'Halloran was born in Philadelphia, Pennsylvania. Fighting as "Irish" Jack O'Halloran, he was a heavyweight boxing contender active from 1966 to 1974. O'Halloran was undefeated throughout his first 16 professional fights.
During his career, O'Halloran defeated former title contenders Cleveland Williams and Manuel Ramos. He also defeated Danny McAlinden who won the bronze medal for boxing at the 1966 British Empire and Commonwealth Games in Kingston, Jamaica and later became the British and Commonwealth Heavyweight champion. O'Halloran's loses included defeats at the hands of future heavyweight champions George Foreman and Ken Norton.
In 1973, O'Halloran was close to attaining a match against Muhammad Ali when he was knocked out by Jimmy Summerville. This ended his chances to fight Ali. Although O'Halloran went on to defeat Summerville by K.O. in a rematch, with only three more wins and five losses he was never again a serious heavyweight contender.
Retiring from boxing in 1974 with a record of 34-21-2 (17 knockout victories) O'Halloran turned to a career as an actor. He first won the role of Moose Malloy in the 1975 film Farewell, My Lovely featuring Robert Mitchum as Philip Marlowe.
This led to other tough "henchmen" style roles which culminated in the role he is best known for: Non, the menacing-but-mute member of the trio of Kryptonian supervillains banished to the Phantom Zone by Jor-El (Marlon Brando) in Superman: The Movie (1978) and inadvertently released, by the Man of Steel himself (Christopher Reeve), in Superman II (1980).
O'Halloran once stated in an interview that it was his idea to make Non a very child-like character, having difficulty adjusting to his new-found powers and making sounds in the absence of voice. It is also believed that, as an American, O'Halloran was the "odd-man-out", teamed with Terence Stamp (General Zod) and Sarah Douglas (Ursa), whose English accents contributed (in the American psyche) to the menacing presence original director Richard Donner wanted for the Kryptonian outcasts. O'Halloran has also vocally criticized Alexander and Ilya Salkind, the producers of the Superman films, for their mishandling of the franchise, believing that their firing director Richard Donner was a huge blow to the series and was the primary result in the series downturn in quality, a sentiment that was shared by Gene Hackman, who refused to reprise his Lex Luthor role in the third film, and Margot Kidder, who played Lois Lane.
According to O'Halloran, the reaction he gets most often from fans is "My God, he can talk!" He also publicly answers questions sent in to him by IMDB fans on his related forum boards.