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