His work in animated cartoon shorts included The Brave Little Tailor, in which he animated scenes of Mickey Mouse and the king; Mickey and the bear in The Pointer, and German dialogue scenes in the World War II propaganda short Education for Death (shortly before Thomas enlisted in the Air Force).
In feature films, among the characters and scenes Thomas animated were the dwarfs crying over Snow White's "dead" body, Pinocchio singing and dancing at the marionette theatre, Bambi and Thumper having fun on the ice, Lady and the Tramp eating spaghetti on their date, the three fairies in Sleeping Beauty, Merlin and Arthur as squirrels and the "wizard's duel" between Merlin and Madam Mim in The Sword in the Stone (in which he was paired with animator Milt Kahl to great effect), King Louie in The Jungle Book (the song number "I Wan'na Be Like You" featuring King Louie and Baloo the Bear dancing, which re-teamed him with Kahl), the dancing penguins in Mary Poppins, and Winnie The Pooh and Piglet in the Winnie The Pooh featurettes. Thomas was directing animator for several memorable villains, including the evil stepmother Lady Tremaine in Cinderella, The Queen of Hearts in Alice in Wonderland,Captain Hook in Peter Pan and Aunt Sarah in Lady and the Tramp.
He retired from Disney on January 31, 1978.
Thomas co-authored, with fellow Disney legend Ollie Johnston, the comprehensive book The Illusion of Life, first published by Abbeville Press in 1981. Regarded as the definitive resource book on traditional hand-drawn character animation (particularly in the Disney style), the book has been republished numerous times, and is widely considered "the bible" among character animators. The book summarised the Disney approach to animation through the so-called 12 basic principles of animation.
Thomas and Johnston were also profiled in the 1995 documentary Frank and Ollie, which screened at the 20th Toronto International Film Festival, directed by Thomas's son Theodore Thomas. The film profiled their careers, private lives, and the personal friendship between the two men.
Thomas's last appearance in an animated film before his death was in The Incredibles (directed by Brad Bird), although he voiced a character, rather than animating one. Frank and his friend and colleague Ollie Johnston voiced and were caricatured as two old men saying "That's old school..." "Yeah, no school like the old school." The pair had previously been heard, and caricatured, as the two train engineers in Bird's The Iron Giant. Frank Thomas died in Flintridge, California at age 92.
The 2001 biography Walt Disney's Nine Old Men & The Art of Animation by John Canemaker (ISBN 0-7868-6496-6) chronicles Thomas' life.
On the Animation Podcast, Disney director John Musker discussed Frank Thomas, and mentioned that at one time, fellow animation great Chuck Jones had christened Thomas the "Laurence Olivier of animators".