The film is sometimes called "An RKO Radioactive Picture." It was filmed near a nuclear test site, and the set was contaminated by nuclear fallout. Photographs exist of John Wayne holding a Geiger counter. After location shooting, contaminated soil was transported back to Hollywood in order to match interior shooting done there. Over the next 20 years, many actors and crew members developed cancer. "People" Magazine researched the cast and crew's health for an article. By the time it was published, in November 1980, 91 of the 220 cast and crew members had developed cancer. Forty-six had died, including John Wayne, Susan Hayward, Pedro Armendáriz (who shot himself soon after learning he had terminal cancer), Agnes Moorehead, John Hoyt and director Dick Powell. Lee Van Cleef had throat cancer, but died of a heart attack. The count did not include several hundred local Native Americans who played extras, or relatives of the cast and crew who visited the set, including John Wayne's son Michael Wayne. The article quoted the reaction of a scientist from the Pentagon's Defense Nuclear Agency to the news, "Please, God, don't let us have killed John Wayne". As of June 2011, the "People" article is available in its archive online.
Eccentric billionaire Howard Hughes backed the film financially. He later paid an extra twelve million dollars for every existing copy because of guilt; he paid to ship sixty tons of soil contaminated with radiation to Hollywood for retakes. He kept a tight hold on the film, not even allowing it to be shown on television, for years. Universal bought the rights to the film in 1979, and according to "The Hollywood Reporter", it had not been seen by the public for 21 years prior to the purchase (allegedly, Paramount obtained reissue rights in 1974).
John Wayne regretted playing Temujin so much that he visibly shuddered whenever anyone mentioned the film's name. He once remarked that the moral of the film was "not to make an ass of yourself trying to play parts you're not suited for."
The movie was filmed not long after the atom-bomb tests in the Yucca Flats area of Nevada, where 11 bombs were tested; although the area was in Nevada, the Utah location where the film was shot, Snow Canyon (not far from the town of St. George), was downwind from the test site and much of the radiation fallout from those tests landed in Snow Canyon. Many of the film's cast and crew received high doses of radiation while shooting at that location (and after the production returned to Hollywood from the location, 60 tons of soil were shipped to the studio so that interior shots and retakes could match the exterior location shots; the studio was unaware that this soil was also contaminated with radiation). Many people involved in the production knew about the radiation--there's a picture of John Wayne himself operating a Geiger counter during the filming--but no one took the threat seriously at the time. Thirty years later, however, half the residents of St. George had contracted cancer, and veterans of the production began to realize they were in trouble. Actor Pedro Armendáriz developed cancer of the kidney only four years after the movie was completed, and later shot himself when he learned his condition was terminal. Howard Hughes was said to have felt "guilty as hell" about the whole affair, although apparently it never occurred to anyone to sue him. For various reasons he withdrew "The Conqueror" from circulation, and for years thereafter the only person who saw it was Hughes himself, who screened it night after night during his paranoid last years.
According to "The Hollywood Hall of Shame," the screenplay was written with Marlon Brando in mind for the lead. John Wayne was about to make the last film of a three-picture deal for RKO Radio Pictures, and Dick Powell had been assigned to direct. They were going over various scripts in Powell's office when Powell was called away for a few minutes. When he returned, he found Wayne enthusiastically looking over the screenplay for "The Conqueror", which Powell had intended to throw away. Powell tried to talk him out of it, but Wayne insisted that it was the film he wanted to make. As Powell later said, "Who am I to turn down John Wayne?"
The character Wang Khan, played by Thomas Gomez, is also known in history books as Ong Khan, Togrul, Toghrul, or Toghril. He was a member of the little-known Nestorian Christian sect, and European missionaries often referred to him in their reports as Prester John, a popular name for a hypothetical ideal Christian king.