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