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