Seth Green as Milo (performance capture)

Seth Robert Dusky as Milo (voice) (as Seth Dusky)

Joan Cusack as Mom (voice)

Dan Fogler as Gribble (voice)

Breckin Meyer as Alien (voice)

Elisabeth Harnois as Ki (voice)

Tom Everett Scott as Milo's Dad (voice)

Mindy Sterling as Supervisor (voice)

Julene Renee as Martian (voice)

Raymond Ochoa as Martian Hatchling (voice)

Robert Ochoa as Martian Hatchling (voice)

Ryan Ochoa as Martian Hatchling (voice)

Amber Gainey Meade as Martian

Dee Bradley Baker as Two-Cat (uncredited)

Adam Jennings as Martian

Aaron Rapke as Martian

Jacquie Barnbrook as Martian

Kevin Cahoon as Wingnut

Kirsten Severson as Martian

Matthew Henerson as Martian

Stephen Kearin as Martian