Following his graduation from the University of Iowa and World War II service, Jock Mahoney came to Hollywood as a stuntman. Quickly establishing a reputation as one of the best and most courageous purveyors of his trade, Mahoney graduated to speaking roles in 1946. Billed as Jacques O'Mahoney, he played villains and secondary roles in Republic and Columbia westerns, showed up as a parodied "strong and silent" leading man in a handful of Three Stooges 2-reelers, and, while doubling for Errol Flynn, performed the legendary staircase leap in 1949's The Adventures of Don Juan.
In 1951, Gene Autry hired Mahoney (who was now billing himself as Jack Mahoney) to star in the popular TV western series The Range Rider. This led to leading roles in such features as Overland Pacific (1954), Showdown at Abilene (1956) and I've Lived Before (1956). In 1958, Mahoney starred in another weekly TV western, Yancey Derringer. Two years later he played the villain in a Tarzan picture starring Gordon Scott, succeeding Scott as the "lord of the jungle" in Tarzan Goes to India (1962) -- during the filming of which he fell deathly ill, a fact that is painfully obvious in the completed picture.