Sammy Davis Jr.'s performance as the sinister head rat calls back to and references his days as a member of the "Rat Pack" in the 1960s.
Production lasted eight years, with delays caused both by other Hanna-Barbera projects and because of a test screening containing the first third of early footage, which was thrown out by H-B co-founder Joseph Barbera because he was dissatisfied with the quality of the animation. He aimed for a lavish animation quality to improve upon the studio's typical low-budget projects.
Originally, Heidi was to be voiced by a juvenile performer; this eventually proved an impossible search. The producers talked with 150 actresses for the role, culminating in stage veteran Margery Gray voicing the title character. (She was 42 at the time of production; the film would sit unreleased for 6 years).
Originally, 20 songs were planned for the film in its early stages by songwriters Burton Lane and Sammy Cahn. The final product ended up with only 16 songs.
Roger DeWitt, Jr. (the voice of Peter) was the son of Jean Vander Pyl, who was a veteran Hanna-Barbera voice actress since the studio's beginning.