Jim Carrey Horton (voice)
Joey King Katie (voice)
Isla Fisher Dr. Mary Lou Larue (voice)
Seth Rogen Morton (voice)
Steve Carell Mayor (voice)
Jaime Pressly Mrs. Quilligan (voice)
Jonah Hill Tommy (voice)
Selena Gomez Helga (voice)
Will Arnett Vlad (voice)
Niecy Nash-Betts Miss Yelp (voice)
Teresa Ganzel Additional Voices (voice)
Madison Pettis Additional Voices (voice)
Fletcher Sheridan Jojo (singing voice) (uncredited)
Bob Bergen Additional Voices (voice)
Ariel Winter Additional Voices (voice)
Ken Daurio Screenplay
Jan Rabson Town Cryer (voice)
Dennis Leonard Supervising Sound Editor
Dan Fogler Councilman / Yummo Wickersham (voice)
William H. Frake III Storyboard Artist
John Cygan Who (voice)
Jesse McCartney JoJo (voice)
Madison Davenport Additional Voices (voice)
Chris Meledandri Executive Producer
Dr. Seuss Book
Karen Disher Who Kid (voice), Storyboard Artist
Mickie McGowan Additional Voices (voice)
Bill Farmer Willie Bear (voice)
Carol Burnett Kangaroo (voice)
Chris Wedge Executive Producer
Amy Poehler Sally O'Malley (voice)
Laraine Newman Glummox Mom (voice)
Mac Smith Sound Effects Editor
Cinco Paul Screenplay
Jack Angel Old Time Who (voice)
Josh Flitter Rudy (voice)
J.G. Quintel Additional Storyboarding
Chris Renaud Storyboard Artist
Jeannine Berger Post Production Supervisor
Jess Harnell Another Who (voice)
Jimmy Hayward Obnoxious Who (voice), Director
Debi Derryberry Who Mom (voice)
John Powell Original Music Composer
Rich Moore Additional Storyboarding
Colleen O'Shaughnessey Angela (voice)
Randy Thom Sound Designer, Supervising Sound Editor
Arun Ram-Mohan Lighting Artist
Eric Favela Storyboard Artist
Grace Rolek Additional Voices (voice)
Steve Martino Director
Laura Ortiz Jessica (voice)
Tim Nordquist Wickersham Guard 2 (voice), Editor
Jeff Biancalana Additional Voices (voice), Storyboard Artist
Mona Marshall Additional Voices (voice)
Danny Mann Additional Voices (voice)
Tony Maki Storyboard Artist
Christian Kaplan Casting
Isabella Acres Additional Voices (voice)
Marshall Efron Wickersham Guard 1 (voice)
Frank Marino Additional Storyboarding
Joe Pasquale The Dentist (voice)
Samantha Droke Hildy / Holly (voice)
Cal Brunker Storyboard Artist
Sherry Lynn Additional Voices (voice)
Barry E. Jackson Visual Development
Tansal Arnas Schedule Coordinator
Heather Goldenhersh Who Girl (voice)
Ricardo Curtis Head of Story
Paul Downs Animation
Jason Fricchione Joe (voice)
Edmund Fong Additional Storyboarding
Simon Brown Senior Visual Effects Supervisor