Mahoney was married three times, first to Lorraine O'Donnell, with whom he had two children, Kathleen O'Mahoney and Jim O'Mahoney. He next married Actress Margaret Field on December 11, 1959, in Las Vegas. They had one child, Princess O'Mahoney, born in 1962. Margaret Field already had two children, Richard Field and Sally Field. Mahoney and Field divorced in June 1968. The following year, he married Actress Autumn Russell, who had three children, Carl Botefuhr, Angela Russell and Andrea von Botefuhr. They remained together until his death.