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.