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