Jock Mahoney was born in Chicago, Illinois, but reared in Davenport, Iowa. He entered the University of Iowa in Iowa City 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 he moved to Los Angeles, California, 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 Jock Mahoney, who demanded and received $1,000 for the dangerous stunt. Most of Mahoney’s films of the late 1940s and early 1950s were produced by Columbia Pictures. Like many a Columbia contract player, Mahoney worked in the studio’s two-reel comedies. Beginning in 1947, writer-director Edward Bernds cast Mahoney in slapstick comedies starring The Three Stooges. Mahoney had large speaking roles in these films, and often played his scenes for laughs. In the Western satire Punchy Cowpunchers (1950), Mahoney, striking a heroic pose, would suddenly get clumsy, tripping over something or taking sprawling pratfalls. Beginning in 1950, Columbia management noticed Mahoney’s acting skills and gave him starring roles in adventure serials. He was originally billed as Jacques O’Mahoney, then Jock O’Mahoney.

Jock Mahoney succeeded stuntman Ted Mapes as the double for Charles Starrett in Columbia’s Durango Kid western series. The Durango Kid often wore a mask covering much of his face, which enabled Mahoney to replace Starrett in the action scenes. Mahoney’s daring stunts made it seem that the older Starrett grew, the more athletic he became. 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. When Charles Starrett’s contract ran out in the spring of 1952, Columbia decided to replace him with Mahoney, opposite Starrett’s sidekick Smiley Burnette. The first film was completed but never released; Columbia abandoned the series in June 1952, bringing an end to its long history of B-Western production. 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. In 1959 there was a lost episode shown six years after the series ended. He was billed as Jack Mahoney. The character had no name other than Range Rider. His series co-star was Dick Jones, playing the role of Dick West.

In the 1958 western film Money, Women and Guns, Jock Mahoney played the starring role. The film also starred Kim Hunter. For the 1958 television season, he starred in the semi-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 (‘pa-who-kaht’-chee-wah’) (“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. Jock O’Mahoney starred in 64 feature films. 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. He died of another stroke two days after being involved in an automobile accident in Bremerton, Washington. His ashes were scattered into the Pacific Ocean.