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