The idea was originally conceived in 2001, when Tim Allen was initially cast as the Cat, but he dropped his role due to work on The Santa Clause 2, and the role was later given to Mike Myers. Filming took place in California for three months. While the basic plot parallels that of the book, the film filled out its 81 minutes by adding new subplots and characters quite different from those of the original story, similar to the feature film adaptation of How the Grinch Stole Christmas!.
The Cat in the Hat was released on November 21, 2003 in the United States and grossed over $133 million. Later, Dr. Seuss' widow Audrey Geisel, who owns her husband's works, decided not to allow any further live-action adaptations of Seuss' work. After this, a planned sequel based on The Cat in the Hat Comes Back was cancelled.
Conrad and Sally Walden live in the city of Anville with their single mother Joan Walden, who works for neat-freak Hank Humberfloob. One day, Mr. Humberfloob firmly warns Joan that she will be fired if her house is messy for the upcoming meet and greet party. At the house, Conrad trashes the house, causing their dog Nevins to run down the street in fright. Joan punishes Conrad for a week while re-cleaning the house. Their next door neighbor, Lawrence Quinn, whom Joan is dating to Conrad's dismay, brings back Nevins, and Sally is grateful. When Lawrence leaves, Joan is called back to the office again, leaving the kids with Mrs. Kwan, a lethargic babysitter, and making sure they are forbidden to enter the living room, which is being kept pristine for the upcoming party. Lawrence is constantly on the lookout for any mischief that Conrad is up to, as he wants nothing more than to send him away to military school for the rest of his life, earning Conrad the reputation of "troublemaker", while his sister is characterized as "perfect and well-behaved".
Once their mother leaves, and Mrs. Kwan is falling asleep, Sally and Conrad discover a humanoid, oversized talking Cat in a Hat in their house. The Cat wants them to learn to have fun, though the children's pet Fish doesn't want the cat around while Joan is away. In a series of antics, the Cat ruins Joan's best dress, jumps on the living room's couch, and bakes cupcakes that explode. In the process, he even releases two troublemaking Things from a crate that he explains is actually a portal from their world to his. The Cat tells Conrad that he only has one rule: that he must never open the crate. The Cat tells the Things to fix Joan's dress; however, they end up wrecking the house instead, since they only do the opposite of what is said. Despite the Cat's warning, Conrad picks the lock anyway. When the crate's lock attaches itself to the collar of the family dog, Nevins, which then escapes, the Cat and the kids go out to find it.
Meanwhile, Lawrence is revealed to be a disgusting and unemployed slob who has false teeth and is in financial ruin, showing off the impression as a successful businessman in the hopes of marrying to Joan for her fortune and sponging off of her. Lawrence sees Nevins running across the street and sees that this is an opportunity for Joan to send Conrad to military school as punishment and allow him to move in. They are almost discovered by some children from the nearby birthday party of Sally's former friend, Denise, during which the Cat hides by pretending to be a piñata and is subsequently beaten.
While spying on Nevins, the cat and the kids see Lawrence arrive and take the dog. The Cat and the kids are witness to this and, using the Cat's super-powered car, they follow Lawrence into town but end up crashing the car to a pole. Lawrence goes to see Joan, but the Cat (disguised as a hippie) intervenes and tricks Lawrence into handing over the dog and he and the kids escape. They later see an anxious Lawrence driving home with Joan, but Conrad uses Things 1 and 2 to stall her by posing as police officers, giving them time to get back using Lawrence's car. While distracting them, Lawrence sees the group drive past and races back to the house, telling Joan to meet him there.
During this time, "the mother of all messes" has been emitted from the unlocked crate and enters the house. Lawrence catches the kids out the front and pushes them into the house, where they find it surprisingly immaculate. A hidden Cat then reveals himself to Lawrence who stumbles back in fear sneezing (he is allergic to cats), tearing through a wall and falling off a bottomless cliff, revealing the Cat's world. The trio navigate their way through the oversized house by riding Mrs. Kwan and find the crate sucking up things that disappears forever once gone through. Sally is nearly sucked up but Conrad manages to put the lock back on the crate to save her and the house. The house returns to its normal proportions but immediately falls apart. The Cat then tells the kids that he had planned the whole day, including making not opening the crate his only rule, as he knew Conrad could not resist and also admits he never really lost his magic hat. The kids angrily tell the Cat to leave the house for the destruction he has caused, and then brace themselves for their mother's arrival. However, the Cat happily returns to clean up his mess with a great cleaning contraption much to Conrad and Sally's surprise and delight.
Afterwards, when everything is restored to its original cleanliness, the Cat says goodbye to Conrad and Sally as they plead with him not to go but he departs just as Joan is coming in. Lawrence arrives, thinking he has busted the kids, but when Joan sees the clean house (and a really messy Lawrence), she disbelieves him, and dumps him. He cries, sneezes in his hands, disgusting Joan (it is possible that she has learned of his sloppiness). He asks her to marry him, and she closes the door, and Sally locks him out of the house. Conrad and Sally jump for joy, as Mrs. Kwan falls asleep again.
When her party is successfully completed, Joan and her kids play in the living room by jumping on the couch and having fun (with the cat going out of town with Thing 1 and 2, completing the film as the credits roll).
- Mike Myers as the Cat in the Hat, a large, wise-cracking anthropomorphic cat with a Brooklyn accent. His hat has many magical abilities. He likes to have fun, but often makes mistakes. In the end it is revealed that he was also the narrator.
- Spencer Breslin as Conrad Walden, Joan's friendly and happy, but rule-breaking son, who goes along with the Cat's plans. As a running gag, the Cat constantly calls him names other than Conrad, due to his lack of name in the original book and TV special.
- Dakota Fanning as Sally Walden, Joan's dull, well-behaved, and rule-obeying daughter, who learns to have fun with the Cat.
- Kelly Preston as Joan Walden, Conrad and Sally's mother, a successful real-estate agent.
- Alec Baldwin as Lawrence "Larry" Quinn, the Waldens' selfish, lazy, unemployed next-door neighbor. He is allergic to cats, steals food from the Walden family, and is determined to both marry Joan for her wealth and send Conrad and Sally away.
- Amy Hill as Mrs. Kwan, an elderly Asian woman with a thick Chinese accent that gets hired to watch the kids, though she sleeps through her job. Her weight and sleep serves as a running gag.
- Sean Hayes as Mr. Hank Humberfloob, Joan's boss. He is also the voice of the family Fish.
- Danielle Chuchran and Taylor Rice as Thing One, and Brittany Oaks and Talia-Lynn Prairie as Thing Two; two gibbering trouble-making creatures that the Cat brings in with him. Dan Castellaneta provided the voices for both Things.
- Steven Anthony Lawrence as Dumb Schweitzer
- Paris Hilton as a female club-goer
- Bugsy as Nevins, the Waldens' pet dog. Frank Welker provided his voice.
- Candace Dean Brown as a secretary who works for the Humberfloob Real Estate.
- Daran Norris as the Announcer
- Clint Howard as Kate the Caterer; makes brownies, cakes, and pies for parties, weddings, funerals, and graduations.
- Paige Hurd as Denise, who doesn't speak to Sally anymore, not long after she talked back to her. She never invited Sally to her birthday party either since Sally earlier stated that she told Denise not to speak to her anymore, which is most likely.
- Stephen Hibbert as Jim McFinnigan. Touching Humberfloob's hand, Jim gave him a handshake which results in being fired.
- Roger Morrisey as Mr. Vompatatat
- Victor Brandt as the Narrator, who tells the story; he is revealed to be the Cat using a voice-changer at the end.