In the role of the Cat in the Hat, Mike Myers portrays a humanoid and anthropomorphic cat standing at a height of 6 feet. This character is known for his witty remarks and dons an extra-large red bow-tie along with a red-and-white striped top-hat which is equipped with numerous comical devices.

Sally Walden is Joan's 8-year-old daughter, who tends to be unexciting and a bit domineering, but well-mannered and obedient to rules. She is also Conrad's younger sibling.

Sally's older brother and the 12-year-old son of Joan, Conrad Walden is known for his troublesome and disobedient behavior, often causing destruction.

Fish is a talking, anthropomorphic orange fish who serves as the voice of reason and morality in contrast to the mischievous Cat. The Fish is portrayed as overly cautious and fearful, often warning Conrad and Sally about the dangers of the Cat's antics. The character is voiced by actor Sean Hayes in the film.

Two mischievous and nonsensical creatures are introduced by the Cat. These troublemakers are known as "the Things," and their voices are provided by Dan Castellaneta.

An elderly woman of Taiwanese origin who is hired to look after the children. However, she is depicted as overweight and often sleeping on the job, which becomes a recurring joke throughout the film.

The mother of Conrad and Sally is a single parent who works as a real estate agent and is depicted as a workaholic in the movie.

In the movie, he is is portrayed as the Waldens' neighbor, who is both pompous and indolent. He remains jobless and suffers from a cat allergy. Despite stealing food from the Waldens' house, he harbors a desire to marry Joan to gain access to her wealth. Additionally, he wants to send Conrad away to military school to remove him from the picture.

He is Joan's boss and depicted as a germaphobe who appears friendly at first glance but is extremely intolerant when it comes to enforcing workplace rules. He has a zero-tolerance policy and can fire employees for even minor violations. For instance, he detests handshakes due to his fear of germs and will not hesitate to shout at the employees while dismissing them from their job.

He is is a pre-teen boy with a Bronx accent who is depicted as intellectually and socially inferior to the other characters. During a birthday party, when the Cat disguises himself as a piñata to include Sally, this boy hits the Cat in the groin with a wooden bat.

Nevins is the Walden family's pet dog. His character is depicted as a loyal and friendly companion to Conrad and Sally.