Mouse Moody is Judy's pet cat. She is black and white with a pink nose. Her talent is making toast, and she does it at the Fur 'n' Fangs Pet Contest in Judy Moody Gets Famous!.
- In Judy Moody, Judy needs a special pet to put in her Me collage, but realises that she only has Mouse. She and her family go to Fur 'n' Fangs to buy a special pet, but all they could get is a toy mouse for Mouse. When they go to pay for it, she notices a Venus flytrap on the counter, hears facts about it from the pet shop lady, and accepts to buy it.
- In Judy Moody Gets Famous!, she calls Mouse and hugs her, saying that she'll be the most famous cat in the world for the Fur 'n' Fangs Pet Contest. She then takes Mouse to Fur 'n' Fangs for the contest and Mouse does her talent of making toast, but when they take their picture of second place to put in the newspaper, Judy's elbow only appears in the picture.
- In Judy Moody Saves the World!, Judy and Rocky eat bananas and attempt to put them in the compost to make soil, and give some bananas to Mouse. Stink is disgusted by the sight of banana smoosh on Mouse's whiskers and tells Judy that he's telling Mom about it.
- In Judy Moody: The Doctor Is In!, Mouse's hair is mentioned to be stuck on a thermometer when Mom takes it out of her bag.
- In Judy Moody Declares Independence, Judy feeds Mouse as one of her jobs of being responsible and installs a sign that says "Judy Moody fed Mouse here" on the floor next to Mouse's food bowl.
- In Judy Moody Goes to College, Mouse appears in Judy's room when Judy tries to get her to roll on the rug to make it hairy like the rug in Chloe's bedroom at college. She then appears in the TP Club Tent when Judy does yoga, doing the down-dog. Mouse does the same thing.
- In Judy Moody and the NOT Bummer Summer (same as the movie), Mouse appears in Judy's room and for the third time, Judy scratches her ears (in the movie), and paces up and down her room talking to her (in the book) that Aunt Opal might look ugly and evil and make Judy and Stink eat fish guts for breakfast.