John Goodman as James P. "Sulley" Sullivan, a large, blue furry monster with horns and purple spots. Even though Sully excels at scaring children, he is kindhearted and thoughtful by nature. At the film's beginning, Sully has been the "Best Scarer" at Monsters, Inc. for several months running.
Billy Crystal as Michael "Mike" Wazowski, a short, round green monster with a single big eyeball and skinny limbs. Mike is Sulley's stationrunner and coach on the scare floor, and the two are close friends and roommates. Mike is charming and generally the more organised of the two, but is prone to neurotics and his ego sometimes leads him astray. He is dating Celia Mae, who calls him "Googly-Bear". He makes cameo appearances in Finding Nemo, Cars, WALL-E, and Toy Story 3.
Mary Gibbs as Mary "Boo", a three-year-old[2][3][4] human girl who is unafraid of any monster except Randall, the scarer assigned to her door. She refers to Sulley as "Kitty". The book based on the film gives Boo's "real" name as Mary Gibbs, the name of her voice actress. In the film, one of Boo's drawings is covered with the name "Mary".
Steve Buscemi as Randall Boggs, a purple eight-legged lizard monster with a chameleon-like ability to change skin color and blend in completely with his surroundings. He is a snide and preening character who makes himself a rival to Mike and Sulley in scream collection.
James Coburn as Henry J. Waternoose III, an arthropodic monster with a crab-like lower body. Waternoose is the CEO of Monsters, Inc., a job passed down through his family for three generations. He acts as a mentor to Sully, holding great faith in him as a scarer.
Jennifer Tilly as Celia Mae, a gorgon-like monster with one eye and tentacle-like legs. Celia is the receptionist for Monsters, Inc. and Mike's girlfriend.
Bob Peterson as Roz, a slug-like monster with a raspy voice reminiscent of Selma Diamond's who administrates for Scarefloor F where Sully, Mike and Randall work. At the end of the movie, it revealed that Roz is the "number 1" agent of the CDA, and has worked undercover at Monsters, Inc. for about 2 years.
Frank Oz as Jeff Fungus, Randall's red-skinned, three-eyed assistant. Jeff is generally incompetent and is often bullied by Randall.
Dan Gerson as Smitty and Needleman, two goofy adolescent monsters with cracking voices who work as janitors and operate the Door Shredder when required. The pair of them idolize Sully, and are generally seen as a nuisance by Mike.
Steve Susskind as Jerry Slugworth, a red seven-fingered monster who manages Scare Floor F and is a good friend of Waternoose.
Bonnie Hunt as Ms. Flint, a female monster who trains new monsters to scare children.
Jeff Pidgeon as Thaddeus "Phlegm" Bile, a trainee scarer for Monsters, Inc.
Samuel Lord Black as George Sanderson, a chubby, oranged-furredd monster with a sole horn on top of his head. A running gag throughout the film involves George repeatedly making contact with artifacts of human clothing (socks and the like cling to his fur via static), which prompts his scare coach to trigger "23–19" incidents with the CDA resulting in him mobbed and shaved bald.
Phil Proctor as Charlie, George's assistant with sea-green skin and tendrils for limbs. He is friend with George, Mike and Sully but is quick to call the CDA on his scarer at the drop of a hat.
Joe Ranft as Peter "Claws" Ward, a blue monster with razor-sharp claws and horrifying breath.
