The authorship of the screenplay for Citizen Kane, the 1941 American motion picture that marked the feature film debut of Orson Welles, has been one of the film's long-standing controversies. With a story spanning 60 years, the quasi-biographical film examines the life and legacy of Charles Foster Kane, played by Welles, a fictional character based in part upon the American newspaper magnate William Randolph Hearst and Chicago tycoons Samuel Insull and Harold McCormick. A rich incorporation of the experiences and knowledge of its authors, the film earned an Academy Award for Best Original Screenplay for Herman J. Mankiewicz and Welles.

More works of conjecture … have been written on the creation of the screenplay for Citizen Kane than Orson and Herman were ever able to cram between those blue covers.

— Joseph Cotten

Screenwriter Herman J. Mankiewicz was a notorious personality in Hollywood. "His behavior, public and private, was a scandal," wrote John Houseman. "A neurotic drinker and a compulsive gambler, he was also one of the most intelligent, informed, witty, humane and charming men I have ever known." Orson Welles told Peter Bogdanovich that "Nobody was more miserable, more bitter, and funnier than Mank ... a perfect monument of self-destruction. But, you know, when the bitterness wasn't focused straight at you, he was the best company in the world."

Welles admired Mankiewicz, and had met him in New York in 1938 at the time of the Mercury Theatre's Broadway successes. In September 1939 Welles visited Mankiewicz while he was hospitalized in Los Angeles after a car accident, and offered him a job writing scripts for the Mercury Theatre's show on CBS radio, The Campbell Playhouse. "I felt it would be useless," Welles said later, "because of Mank's general uselessness many times in the studios. But I thought, 'We'll see what he comes up with.'" Mankiewicz proved very useful, particularly working with Houseman as editor, and wrote five scripts for Campbell Playhouse shows broadcast between November 12, 1939, and March 17, 1940. Houseman and Welles were partners in the Mercury Theatre, but when Mercury productions moved from New York to California the partnership ended and Houseman became an employee, working primarily as supervising editor on the radio shows. In December 1939, after a violent quarrel with Welles over finances, Houseman resigned and returned to New York. To Welles, his departure was a relief.

In late December, the executive board of RKO Pictures all but ordered studio chief George J. Schaefer to stop paying salaries to the Mercury Productions staff until Welles submitted an acceptable script and set a start date for filming. Over the next five weeks Welles spent many long evenings brainstorming plot ideas in the bedroom of the small rented house where Mankiewicz was in traction with his shattered leg. Mankiewicz testified in a court proceeding a few years later^[specify] that the idea for the film began with a March of Time-style sequence that set out the life of a particular character whose life would then be the subject of the film. Welles later said the germinal idea was to create a posthumous portrait of a man through many points of view, in the recollections of those who knew him. "I'd been nursing an old notion," Welles told Bogdanovich, "the idea of telling the same thing several times—and showing exactly the same thing from wholly different points of view. Basically, the idea Rashomon used later on. Mank liked it, so we started searching for the man it was going to be about. Some big American figure—couldn’t be a politician, because you’d have to pinpoint him. Howard Hughes was the first idea. But we got pretty quickly to the press lords."

Welles and Mankiewicz soon settled on the idea of using newspaper magnate William Randolph Hearst as their central character. By late January their disagreements over plot details were more frequent and their collaboration less creative. "That's why I left him on his own, finally," Welles said later, "because we'd started to waste too much time haggling." In February 1940, Welles arranged a lunch at New York's 21 Club and persuaded Houseman to return to California. He was hired to supervise Mankiewicz as he put a rough draft on paper.

On February 19, 1940, Mankiewicz signed a contract with Mercury Productions to work on the screenplay. He would receive $1,000 a week for his work as long as he was not "incapacitated by illness or any other reasons", and would be paid a $5,000 bonus on delivery of the script. Mankiewicz was to receive no credit for his work as he was hired as a script doctor; a similar clause was present in the writers' contracts for The Campbell Playhouse. Mankiewicz was advised by his agents, Columbia Management of California, and before he signed the contract it was again made clear that all credit for his work belonged to Welles and the Mercury Theatre, the "author and creator". Welles hesitated to officially agree to give Mankiewicz credit. His contract with RKO stated that the film would be produced, directed, performed and written by Welles, and his "boy wonder" persona had great publicity value for the studio. Welles's attorney, Arnold Weissberger, did not want to give RKO any cause to break Welles's contract should they wish to promote the film as solely his work. The contract with Mankiewicz left the matter of his receiving credit open; Welles's correspondence with his lawyer indicated that he did not wish to deny credit to Mankiewicz.