According to a Henry Fonda interview seen on the DVD version of this movie, many locals of Jackson Hole, Wyoming were used as extras for scenes in the movie, such as the graduation of Clayboy.
In their book "How Underdog Was Born...", W. Watts Biggers and Chad Strover reveal that seeing Wally Cox's performance in this movie inspired them to ask him to voice their newly created character, Underdog.
Maureen O'Hara's real-life daughter, Bronwyn FitzSimons, plays the part of the college dean's secretary.
The film debut role of Barbara McNair is somewhat incongruous, as the mountain community is clearly 100% white. She is not involved in the plot, nor does she have any dialogue interchange with any of the actors. She is only credited as "Graduation Singer", and is never seen or referred to again. Warner Brothers, the production company and her record label at the time, was likely trying to jump start her screen career.