Jim Carrey
Horton (voice)
Acting
Steve Carell
Mayor (voice)
Acting
Carol Burnett
Kangaroo (voice)
Acting
Will Arnett
Vlad (voice)
Acting
Seth Rogen
Morton (voice)
Acting
Dan Fogler
Councilman / Yummo Wickersham (voice)
Acting
Isla Fisher
Dr. Mary Lou Larue (voice)
Acting
Jonah Hill
Tommy (voice)
Acting
Amy Poehler
Sally O'Malley (voice)
Acting
Jaime Pressly
Mrs. Quilligan (voice)
Acting
Charles Osgood
Narrator (voice)
Acting
Josh Flitter
Rudy (voice)
Acting
Niecy Nash-Betts
Miss Yelp (voice)
Acting
Jesse McCartney
JoJo (voice)
Acting
Shelby Adamowsky
Hedy / Hooly (voice)
Acting
Jack Angel
Old Time Who (voice)
Acting
Caitlin Rose Anderson
Helen (voice)
Acting
Emily Anderson
Heather (voice)
Acting
Jan Rabson
Town Cryer (voice)
Acting
John Cygan
Who (voice)
Acting
Jess Harnell
Another Who (voice)
Acting
Debi Derryberry
Who Mom (voice)
Acting
Samantha Droke
Hildy / Holly (voice)
Acting
Karen Disher
Who Kid (voice)
Acting
Marshall Efron
Wickersham Guard 1 (voice)
Acting
Bill Farmer
Willie Bear (voice)
Acting
Jason Fricchione
Joe (voice)
Acting
Heather Goldenhersh
Who Girl (voice)
Acting
Selena Gomez
Helga (voice)
Acting
Jimmy Hayward
Obnoxious Who (voice)
Visual Effects
Joey King
Katie (voice)
Acting
Christina Martino
Heidi / Haley (voice)
Acting
Ellie Martino
Hanna (voice)
Acting
Laraine Newman
Glummox Mom (voice)
Acting
Tim Nordquist
Wickersham Guard 2 (voice)
Editing
Colleen O'Shaughnessey
Angela (voice)
Acting
Laura Ortiz
Jessica (voice)
Acting
Joe Pasquale
The Dentist (voice)
Acting
Connor Anderson
Who Child #1 (voice)
Acting
Isabella Acres
Additional Voices (voice)
Acting
Bob Bergen
Additional Voices (voice)
Acting
Jeff Biancalana
Additional Voices (voice)
Art
Madison Davenport
Additional Voices (voice)
Acting
Teresa Ganzel
Additional Voices (voice)
Acting
Kathy Graves Toon
Additional Voice (voice)
Editing
Sherry Lynn
Additional Voices (voice)
Acting
Danny Mann
Additional Voices (voice)
Acting
Mona Marshall
Additional Voices (voice)
Acting
Mickie McGowan
Additional Voices (voice)
Acting
Madison Pettis
Additional Voices (voice)
Acting
Grace Rolek
Additional Voices (voice)
Acting
Jennessa Rose
Additional Voices (voice)
Acting
Ariel Winter
Additional Voices (voice)
Acting
Fletcher Sheridan
Jojo (singing voice) (uncredited)
Acting