During his boxing career, O'Halloran defeated former title contenders Cleveland Williams and Manuel Ramos. He also defeated Danny McAlinden, who won a bronze medal in boxing at the 1966 British Empire and Commonwealth Games in Kingston, Jamaica and later became the British and Commonwealth Heavyweight Champion. O'Halloran's losses included defeats to Joe Bugner, Ron Lyle, and 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 ex-convict Moose Malloy in the 1975 film Farewell, My Lovely, featuring Robert Mitchum as private eye Philip Marlowe.

O'Halloran has also played supporting roles in King Kong (1976), March or Die (1977), The Baltimore Bullet (1980), Dragnet (1987), Hero and the Terror (1988), Mob Boss (1990), and The Flintstones (1994).

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 (1978) and inadvertently released by Superman in Superman II (1980).

In an interview with Starlog Magazine in 2006, O'Halloran stated that he and Christopher Reeve did not get along during the making of Superman II. On one occasion, he had Reeve against a wall, but Richard Donner intervened and dissuaded him from hitting Reeve. (He also later discussed this incident on the How Did This Get Made? podcast's Episode 24.1.) Despite the clash between them, O'Halloran stated that his heart went out to Reeve after his 1995 accident, and commended him for helping others with spinal cord injuries.

In 2008, O'Halloran announced plans to partner with veteran Hollywood executive Jay Samit to create Long Beach Studios, a chain of film studio facilities throughout the United States.

The California Boxing Hall of Fame has listed O'Halloran as one of its inductees of the 2009 HOF class.

In 2010, O'Halloran released Family Legacy. The book also outlines O'Halloran's relationship with his claimed father, a former boss of the Gambino crime family, Albert Anastasia.

O'Halloran was born in Philadelphia, and was raised by his mother, Mary, and stepfather, Peter Paul Patrick O’Halloran. He lived in Runnemede, New Jersey. Fighting as "Irish" Jack O'Halloran from Boston, he was a heavyweight boxing contender active from 1966 to 1974. The 6-foot, 6-inch O'Halloran was undefeated throughout his first 16 professional fights.