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, 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. Often cast alongside heroine Christine McIntyre, Mahoney appeared in the Stooge films Out West (1947), Squareheads of the Round Table (1948) (and its remake, Knutzy Knights (1954)), Fuelin’ Around (1949), and Punchy Cowpunchers (1950). In the Stooge films, Mahoney—striking a heroic pose—would suddenly get clumsy, tripping over something or taking sprawling pratfalls.

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.

In 1948, Mahoney auditioned to play Tarzan after the departure of Johnny Weissmuller, but the role went to Lex Barker.

Beginning in 1950, Columbia management noticed Mahoney’s acting skills and gave him starring roles in adventure serials. He 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.

He next married actress Margaret Field in 1952. Their daughter, Princess O’Mahoney, was born 6 months later. Margaret Field already had two young children, Richard Field and Sally Field, from her first marriage. Mahoney and Field divorced in June 1968. His stepdaughter Sally has since become a well-known Academy Award-winning television and film actress. In her 2018 memoir In Pieces she states that Mahoney subjected her to sexual abuse throughout her childhood.

Mahoney was cast as an engineer, Andy Prentis, in the 1954 episode, “Husband Pro-Tem,” on the syndicated anthology series, Death Valley Days, hosted by Stanley Andrews. In the story line, Prentis is hired by a railroad executive, Alonzo Phelps (Howard Negley) (1898–1983) to negotiate a private agreement with the Indian Chief Black Hawk (Lane Bradford) so that a railroad can be constructed across Indian lands. In his assignment, Prentis soon romantically tangles with Phelps’ daughter, Evelyn (Gloria Marshall) In February 1956, Mahoney co-starred with his wife Margaret Field in the Death Valley Days episode “Swamper Ike”.

In the 1958 Western film Money, Women and Guns, Mahoney played the starring role. The film also starred Kim Hunter.

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.

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, a lost episode was 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 1960, he appeared as Coy Banton, a villain, in Tarzan the Magnificent, starring Gordon Scott. Mahoney’s strong presence, work ethic, and lean (6 foot, 4 inch, 220 pound) frame impressed producer Sy Weintraub, who wanted a “new look” for the fabled apeman.

In 1960, Mahoney guest-starred in the Rawhide episode “Incident of the Sharpshooter”. He also appeared in television guest-starring roles on such series as Batman, the Ron Ely Tarzan series, Hawaii Five-O, Laramie, and The Streets of San Francisco.

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. When this film was released, Mahoney, at 44, became the oldest actor to play the jungle king, surpassing Weissmuller and P. Dempsey Tabler, a record that still stands. 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 1973, he suffered a stroke at age 54 while filming an episode of Kung Fu, but he recovered.

Sally Field, Burt Reynolds, and Brian Keith starred in the 1978 film Hooper, in which both Reynolds and Keith played Mahoney-like stuntmen. Keith’s character, retired, was named Jocko, and was the father of Field’s character.

In 1979, Mahoney married actress Autumn Russell, who had three children, Carl Botefuhr, Jr., Angela Botefuhr, and Andrea Botefuhr. They remained together until his death.

In 1981, Mahoney returned to the Tarzan film series as the stunt coordinator on the John Derek-directed remake of Tarzan, the Ape Man. He was billed as “Jack O’Mahoney”.

Mahoney died of a second stroke at age 70 on December 14, 1989, two days after being involved in an automobile accident in Bremerton, Washington. His ashes were scattered into the Pacific Ocean.

A tribute to Mahoney entitled “Coming Home” was published on the website of marksman Joe Bowman of Houston, a close Mahoney friend. On February 6, 1990, the poem was read at a memorial tribute to Mahoney held at the Sportsmen’s Lodge in Studio City, California. More than 350 attended, included Bowman. The reading was conducted by Mahoney’s widow, Autumn O’Mahoney.