The movie is set in a fictionalized fantasy version of Ireland and is about a unicorn, Amalthea, trying to find out what happened to the last of her species after discovering that she is the last unicorn in the world, with the help of Schendrick and Molly Grue.
- Mia Farrow as the Unicorn/Lady Amalthea
- Alan Arkin as Schmendrick
- Jeff Bridges as Prince Lir
- Tammy Grimes as Molly Grue
- Robert Klein as The Butterfly
- Angela Lansbury as Mommy Fortuna
- Christopher Lee as King Haggard
- Keenan Wynn as the Harpy Celaeno\Captain Cully
- Paul Frees as Mabruk\The Tree\The Cat
- Rene Auberjonois as The Skull
- Brother Theodore as Ruhk
- Edward Peck as Jack Jinly
- Jack Lester as Hunter #1
- Kenneth Jennings as Hunter #2
- Don Messick as Additional Voices
Peter S. Beagle stated that there had been interest in creating a film based on the book "early on".
Those who expressed interest included Lee Mendelson and Bill Melendez, though Beagle had been convinced by one of their partners' wives that they were "not good enough" and former 20th Century Fox animator Les Goldman.
At the time, Beagle believed that "animated was the only way to go" with regard to the film, and had never thought of making it into a live-action film. Rankin/Bass had been the last studio that the film's associate producer, Michael Chase Walker, approached, and Beagle was "horrified" when he was informed that they had made a deal with Walker.
Beagle stated that he has "…come to feel that the film is actually a good deal more than I had originally credited", and went on to say, "There is some lovely design work – the Japanese artists who did the concepts and coloring were very good. And the voice actors do a superb job in bringing my characters to life…"
While Rankin/Bass provided the film's dialogue and story based on Beagle's work, the animation was done by the studio Topcraft.
The studio would later be hired by Hayao Miyazaki to work on Nausicaä of the Valley of the Wind, and their core members eventually went on to form Studio Ghibli.
According to Beagle, the final film ended up being "remarkably close" to his original script, although one scene at the end involving an encounter with a princess was "animated but eventually cut".
The movie debuted at #6 at the box office, grossing $2,250,000 during its opening weekend, coming in behind films such as the re-release of Star Wars Episode V: The Empire Strikes Back, First Blood, Creepshow, E.T. the Extra-Terrestrial and An Officer and a Gentleman.
Domestically, it made $6,455,330.
"The Last Unicorn" was given a rating of 60% on Rotten Tomatoes.
Janet Maslin from the New York Times called the movie "an unusual children's film in many respects, the chief one being that it is unusually good. [...] features a cast that would do any live-action film proud, a visual style noticeably different from that of other children's fare, and a story filled with genuine sweetness and mystery" and also said that "no one of any age will be immune to the sentiment of the film's final moments, which really are unexpectedly touching and memorable".
Beagle himself called the film "magnificent" in comparison to J.R.R. Tolkien's The Lord of the Rings, for which he also wrote the screenplay.
A 1982 Variety reviewer praised the script and voice acting, but wasn't impressed by the film's animation, saying, "However vapid the unicorn may appear to the eye. Mia Farrow's voice brings an almost moving plaintive quality to the character." The review also praised the vocal talents of Arkin, Lee and Frees.