Mayzie, a lazy, irresponsible bird, convinces an elephant named Horton to sit on her egg while she takes a short "break". However, free of her commitment, she relocates to Palm Beach.
As Horton sits in the nest on top of a tree, he is exposed to the elements, laughed at by his jungle friends, captured by hunters, forced to endure a terrible sea voyage, and finally placed in a traveling circus. Despite his hardships and Mayzie's clear intent not to return, Horton refuses to leave the nest because he insists on keeping his word, often repeating, "I meant what I said, and I said what I meant. An elephant's faithful, one hundred percent!"
The traveling circus arrives near Mayzie's new Palm Beach residence. She visits the circus just as the egg is due to hatch, after 51 weeks in Palm Beach, and demands that Horton return it, without offering him a reward. However, when the egg hatches, the creature that emerges is an "elephant-bird", a cross between Horton and Mayzie, and Horton and the baby are returned happily to the jungle.
- The scene of the Peter Lorre fish seeing Horton on the boat and asiding to the audience, "Well, now I've seen everything" and shooting himself in the head was censored when it aired on TBS, TNT, The WB, Cartoon Network (barring its appearance on The Bob Clampett Show) and Boomerang. However, this scene was left uncut on MeTV airings.
- The version of this cartoon shown on the TNT special In Search of Dr. Seuss re-dubs and re-edits the entire short, and also removes the opening and ending title cards from the Blue Ribbon reissue.
- Clampett's unit didn't use the customary storyboard, but added ideas for the cartoon in Clampett's copy of Seuss' book. Among the elements they added were:
- The opening paragraph, starting with "Now once in a jungle . . ." and ending with " . . . up in her tree."
- A nonsense song of the time, "The Hut-Sut Song" by Horace Heidt - Words and music by Leo V. Killion, Ted McMichael & Jack Owens, sung by Horton and his son.
- A fish caricature of Peter Lorre who commits suicide after seeing Horton on a boat
- Mayzie's breathy Katharine Hepburn impression.
- Mayzie unsuccessfully attempts to lure Horton using sex appeal.
- Several other bits of dialogue are omitted or rewritten.
- This is the only Looney Tunes or Merrie Melodies short based on a Dr. Seuss book.
- The original titles have been found but have not yet been restored for DVD.
- "Horton Hatches the Egg" was adapted from the Dr. Seuss book of the same name. Horton and the Peter Lorre fish were voiced by Kent Rogers, Mayzie and the elephant bird were voiced by Sara Berner, Frank Graham narrated, and Mel Blanc performed most of the other voices.
- The Turner "dubbed version" print retains the 1941-55 MM end music (applies to both the USA and EU prints).
- Production on this cartoon began in August of 1941.
- In the book from Dr. Seuss as well as other animated screen adaptations (including the 2008 CGI-animated film by Blue Sky Studios), Horton's skin is gray.
- Clocking at a total run-time of about 9 minutes 50 seconds (excluding its original technical credits), this is one of the longest-running Looney Tunes and Merrie Melodies shorts ever produced.
- The AAP print uses the 1955-56 Merrie Melodies ending card.
- The LaserDisc print changes the ending title to that of a 1949 or 1953-54 season ending Color Rings scheme plastered with the 1946-55 Looney Tunes music.
- According to Jim Korkis, Dr. Seuss was disappointed about the new additions to the cartoon that weren't in his original book.