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