Jim Carrey Horton (voice)
Joey King Katie (voice)
Isla Fisher Dr. Mary Lou Larue (voice)
Steve Carell Mayor (voice)
Ariel Winter Additional Voices (voice)
Selena Gomez Helga (voice)
Seth Rogen Morton (voice)
Jaime Pressly Mrs. Quilligan (voice)
Jonah Hill Tommy (voice)
Will Arnett Vlad (voice)
Dan Fogler Councilman / Yummo Wickersham (voice)
Niecy Nash-Betts Miss Yelp (voice)
Bob Bergen Additional Voices (voice)
Dr. Seuss Book
Colleen O'Shaughnessey Angela (voice)
Madison Davenport Additional Voices (voice)
Isabella Acres Additional Voices (voice)
Amy Poehler Sally O'Malley (voice)
Madison Pettis Additional Voices (voice)
J.G. Quintel Additional Storyboarding
Laraine Newman Glummox Mom (voice)
Bill Farmer Willie Bear (voice)
Jesse McCartney JoJo (voice)
Rich Moore Additional Storyboarding
Ken Daurio Screenplay
Carol Burnett Kangaroo (voice)
Jack Angel Old Time Who (voice)
Teresa Ganzel Additional Voices (voice)
Jess Harnell Another Who (voice)
Debi Derryberry Who Mom (voice)
Mickie McGowan Additional Voices (voice)
Jon Vitti Story Consultant
Randy Thom Sound Designer, Supervising Sound Editor
Chris Renaud Story Artist
Cinco Paul Screenplay
Eric Favela Story Artist
Josh Flitter Rudy (voice)
Joe Pasquale The Dentist (voice)
Cal Brunker Story Artist
Danny Mann Additional Voices (voice)
Karen Disher Who Kid (voice), Story Artist
Chris Wedge Executive Producer
Grace Rolek Additional Voices (voice)
John Cygan Who (voice)
Mona Marshall Additional Voices (voice)
Dennis Leonard Supervising Sound Editor
Jan Rabson Town Cryer (voice)
Chris Meledandri Executive Producer
Crystal Chesney-Thompson Additional Storyboarding
Samantha Droke Hildy / Holly (voice)
John Powell Original Music Composer
Sherry Lynn Additional Voices (voice)
Galen T. Chu Visual Effects
Fletcher Sheridan Jojo (singing voice) (uncredited)
Jeff Biancalana Additional Voices (voice), Story Artist
Heather Goldenhersh Who Girl (voice)
Jimmy Hayward Obnoxious Who (voice), Director
Marshall Efron Wickersham Guard 1 (voice)
Michael Thurmeier Visual Effects
Ricardo Curtis Head of Story
Steve Martino Director
Jason Fricchione Joe (voice)
Chris Gilligan Additional Storyboarding
Charles Osgood Narrator (voice)
Wesley Mandell Lighting Artist
Michael Knapp Set Designer
Audrey Geisel Executive Producer
Simon Brown Senior Visual Effects Supervisor
Paul Downs Animation
Mac Smith Sound Effects Editor
Edmund Fong Additional Storyboarding
Shelby Adamowsky Hedy / Hooly (voice)