The "mudhole" in which the famous brawl took place wasn't actually made of mud. It was made of a material called bentonite, which is used in the drilling of oil wells and has the consistency of chocolate syrup. According to actor Leo Gordon (the first one to be knocked down it), that scene took a week to shoot.
John Wayne was very annoyed when promotional events were postponed for a week following the assassination of John F. Kennedy. Wayne hated Kennedy and blamed him for losing the Bay of Pigs invasion and for giving in to Soviet demands during the Cuban Missile Crisis.
John Wayne insisted that the role of the weak, insipid Governor be called "Cuthbert H. Humphrey", with the intention that he be seen as a parody of liberal Sen. Hubert H. Humphrey, whom Wayne intensely disliked.
When Batjac, John Wayne's production company, needed completion funds for The Alamo (1960), it borrowed the money from United Artists. The Batjac film library was used as collateral for the loan. Making "McClintock" profitably for United Artists allowed Batjac to reclaim control of the films.
According to producer/son Michael Wayne, in the scene where JW (John Wayne) jumps from a hay loft to a pile of hay in a sitting position, Wayne thought it looked like fun and insisted on doing it himself, instead of a stuntman. Although studios frown on a valuable star doing a potentially dangerous stunt, Wayne eventually was allowed to do it, but it was also shot with a stuntman, just in case.
The failure of the original copyright holder to renew the film's copyright resulted in it falling into public domain, meaning that virtually anyone could duplicate and sell a VHS/DVD copy of the film. Therefore, many of the versions of this film available on the market are either severely (and usually badly) edited and/or of extremely poor quality, having been duped from second- or third-generation (or more) copies of the film.
Maureen O'Hara stated in an interview that she wanted to do the stunt where she is supposed to jump through a plate-glass window. The studio not only would not allow her to do it, but it wouldn't allow even a stunt woman to do it--they insisted it was too dangerous even for a trained stunt woman and said it had to be done by a male stuntman. So the stunt was performed by veteran stunt man Dean Smith wearing a wig and O'Hara's clothes.
In the scene where the Comanches are being outfitted with rifles, it's easy to see that they're Krag Jorgensen carbines, meaning that this film takes place in at least 1896, as the Krag wasn't issued as a standard military rifle until 1894.
Produced by John Wayne's Batjac Productions for United Artists, Wayne's estate retained the rights to the film. In 1994 a legal case determined the film was in the public domain in the U.S., but the music score remained under copyright.
The trivia item below may give away important plot points.
During the scene when GW chases Katherine through town, if you listen carefully, you can hear the melody of "With Cat-Like Tread" from the Gilbert and Sullivan operetta "Pirates of Penzance". It is played in at least 2 different keys, as well as a counter-melody version after the famous spanking scene.