After graduating from Stanford in 1933,[5] he attended Chouinard Art Institute, then joined The Walt Disney Company on September 24, 1934, as employee number 224.

[6] In feature films, among the characters and scenes Thomas animated were the dwarfs crying over Snow White's "dead" body, Pinocchio singing at the marionette theatre, Bambi and Thumper on the ice, Lady and the Tramp eating spaghetti, 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 re-teamed him with Kahl), the dancing penguins in Mary Poppins, and Winnie The Pooh and Piglet in Winnie the Pooh and the Blustery Day and Winnie the Pooh and Tigger Too.

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 James Hook in Peter Pan, and story consultant in Little Nemo: Adventures in Slumberland.

In 2012, Theodore Thomas also directed another short documentary, "Growing up with Nine Old Men", included in the Diamond edition of Disney's Peter Pan DVD.

[9] The 2001 biography Walt Disney's Nine Old Men & The Art of Animation by John Canemaker (ISBN 0-7868-6496-6) chronicles Thomas' life.