After his discharge from the Marine Corps, Mahoney moved to Los Angeles, and for a time was a horse breeder. However, he soon became a movie stuntman, doubling for Gregory Peck, Errol Flynn, and John Wayne. Director Vincent Sherman recalled staging the climactic fight scene in his 1948 film "Adventures of Don Juan" and could find only one stuntman who was willing to leap from a high staircase in the scene. That man was Mahoney, who demanded and received $1,000 for the dangerous stunt.