Hart began his screen career in 1937 with a bit part in Daughter of Shanghai. He continued in a variety of B pictures such as Prison Farm and King of Alcatraz before appearing in two of Cecil B. DeMille’s films The Buccaneer (1938) and North West Mounted Police (1940). His acting career was interrupted when he was drafted into the United States Army in 1941. He rose to the rank of first sergeant in the Coast Artillery, eventually serving in the Philippines. Following military service, he worked frequently for Sam Katzman and was given the lead in the Jack Armstrong (1947) film serial. Hart did stunt work and acted in numerous Westerns and was eventually offered the opportunity to replace Clayton Moore on The Lone Ranger television series. Based on the assumption that the masked character, rather than the actor, was the true star of The Lone Ranger, the program’s producers fired Moore (presumably over salary differences) and replaced him with Hart, who was of a similar build and had a comparable background in westerns. However, the public never truly accepted Hart as the Lone Ranger[citation needed], and by 1954 the producers returned Moore to the role. (According to Clayton Moore’s autobiography, I Was That Masked Man, Moore never knew why he was replaced with John Hart. He also stated that he had not sought a pay increase to continue portraying the Lone Ranger). Hart continued to act in films for more than two decades. He appeared in films of several genres, almost always in supporting roles. In 1955, he starred in The Adventures of Captain Africa, which was originally intended to be a new movie about famous comic book hero The Phantom. However, licensing issues made Columbia re-film the entire serial and re-christen the hero “Captain Africa.”  Hart also had numerous supporting roles in the Highway Patrol TV series.

In 1957, Hart portrayed Nat “Hawkeye” Cutler in the syndicated western series Hawkeye and the Last of the Mohicans, set in New York State in the 1750s. The one-season program was based loosely on the novels of James Fenimore Cooper. It was filmed in Canada and featured some Indians in a favorable image. Interviewed by Tom Weaver in “Western Clippings” magazine (May–June 2000), Hart revealed that he met actress Beryl Braithwaite when she played a role in an episode of the series; they married just days later, with “Hawkeye” co-star Lon Chaney, Jr., the best man at the wedding. In 1965 Hart made two brief appearances on Perry Mason, including the role of title character and murder victim Jamison Selff in “The Case of the Wrathful Wraith.” In The Phynx, released in 1970, Hart played the Lone Ranger alongside Jay Silverheels as Tonto, spoofing their characters. His last theatrical film appearance was in 1981’s The Legend of the Lone Ranger in which he appeared as a newspaper editor. He appeared in the television series Happy Days as the Lone Ranger in the episode “Hi Yo, Fonzie Away” (February 9, 1982). In this episode Fonzie, played by Henry Winkler gets to meet his childhood hero, the Lone Ranger, for his birthday. Hart’s other major late appearance was in an episode of The Greatest American Hero, My Heroes Have Always Been Cowboys, where Hart gives the title character an inspiring speech about heroism. On September 20, 2009, Hart died at his home in Playas de Rosarito, Baja California, Mexico at the age of 91, survived by his wife Beryl, a son and a daughter. His ashes were scattered into the Pacific Ocean.