Jacques Joseph O'Mahoney, known professionally as Jock Mahoney, was an American actor and stuntman. He starred in two Western television series, The Range Rider and Yancy Derringer. He played Tarzan in two feature films and was associated in various capacities with several other Tarzan productions. He was sometimes credited as Jack O'Mahoney or Jock O'Mahoney.

Jock entered the University of Iowa in Iowa City and excelled at swimming and diving, but dropped out to enlist in the United States Marine Corps when World War II began. He served as a pilot, flight instructor, and war correspondent. 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. Most of Mahoney's films of the late 1940s and early 1950s were produced by Columbia Pictures. Like many Columbia contract players, Mahoney worked in the studio's two-reel comedies. Beginning in 1947, he starred with the Three Stooges in their films Out West, Squareheads of the Round Table (and its remake, Knutzy Knights), Fuelin' Around, and Punchy Cowpunchers.

Beginning in 1950, Columbia management gave him starring roles in adventure serials. Mahoney contributed so much to this series that he was awarded featured billing and major supporting roles as well, first as villains and then as sympathetic characters. By 1952 Columbia was billing him as Jack Mahoney. Cowboy star Gene Autry, then working at Columbia, hired Mahoney to star in a television series. Autry's Flying A Productions filmed 79 half-hour episodes of the syndicated The Range Rider from 1951 to 1953.

For the 1958 television season, he starred in the somewhat Western Yancy Derringer series for 34 episodes, which aired on CBS. Yancy Derringer was a gentleman adventurer living in New Orleans, Louisiana, after the American Civil War. He had a Pawnee Indian companion named Pahoo Katchewa ("Wolf Who Stands in Water"), who did not speak, played by X Brands. Pahoo had saved the life of Derringer, and thereafter was responsible for Derringer's life.

In 1962, Mahoney became the 13th actor to portray Tarzan when he appeared in Tarzan Goes to India, shot on location in India. A year later, he again played the role in Tarzan's Three Challenges, shot in Thailand. Dysentery and dengue fever plagued Mahoney during the shoot in the Thai jungles, and he plummeted to 175 pounds. He needed a year and a half to regain his health. Owing to his health problems and the fact that producer Weintraub had decided to go for a "younger look" for the apeman, his contract was mutually dissolved. In the 1980s, Mahoney made guest appearances on the television series B. J. and the Bear and The Fall Guy. During the final years of his life, he was a popular guest at film conventions and autograph shows.

Mahoney died of a second stroke at age 70, two days after being involved in an automobile accident in Bremerton, Washington. His ashes were scattered into the Pacific Ocean.