Mahoney is of French and Irish extraction, with some Cherokee. At the University of Iowa, he was outstanding in swimming, basketball and football. When World War II broke out, he enlisted as a Marine fighter pilot and instructor. In Hollywood, he was a noted stunt man, doubling for Errol Flynn, John Wayne, and Gregory Peck. Gene Autry signed him for the lead in his 78-episode The Range Rider (1951) TV series. He tested to replace Johnny Weissmuller, as Tarzan but lost out to Lex Barker. In 1960, he played the heavy in Gordon Scott's Tarzan the Magnificent (1960), and his part there led Sy Weintraub to hire him as Scott's replacement. In his two Tarzan movies, he did all his own stunts. In Tarzan's Three Challenges (1963), he continued working in spite of dysentery, dengue fever and pneumonia. By this time, Weintraub was looking for a younger Tarzan, envisioning a future TV series. By mutual agreement, his contract with Mahoney was dissolved. After a couple of years regaining his strength and weight, Jock returned to making action films.