The novel on which the movie was based was titled "The Gentleman from Montana", but the state is not specified in the movie
The scenes where James Stewart wanders around in amazement at the Washington monuments were "stolen", since the US Parks Service had denied the studio permission to film near them.
The screenplay was originally purchased by Columbia as a vehicle for Ralph Bellamy, with Harold Wilson slated to produce. Once Frank Capra became the director, the project, planned as a sequel to Mr. Deeds Goes to Town, was entitled "Mr. Deeds Goes to Washington", and was to star Gary Cooper, reprising his role as Longfellow Deeds. Cooper was unavailable for the role, however, and James Stewart was borrowed from MGM. "I knew he would make a hell of a Mr. Smith," Capra said. "He looked like the country kid, the idealist. It was very close to him."
The set for the Senate chamber was constructed on two newly built adjoining stages at Columbia, stage 8 and 9. The set was built almost to scale, and was at that time, the largest set built on a Columbia sound stage.
The state that Jefferson Smith (James Stewart) and Sen. Joseph Harrison Paine (Claude Rains) come from is never mentioned.
The U.S. Senate interior as shown in the film was an extremely accurate reproduction of the real thing.
The Washington press corps were highly indignant at the way they were portrayed in the film. Consequently a great deal of the initial reviews from the capitol were very negative. One of their chief objections was that the film made them all out to be drinking too much.
This film is one of five times that Beulah Bondi portrayed James Stewart's mother. The others are: It's a Wonderful Life, Of Human Hearts and Vivacious Lady, and once on his television series, The Jimmy Stewart Show.
This was reportedly Jean Arthur's favorite film of her own.
To make his voice hoarse for the filibuster scene, James Stewart dried out his throat with bicarbonate of soda. However, both Frank Capra and Stewart revealed in interviews that his throat was periodically swabbed with mercuric chloride.