Brianna Davey said the cat was taken by Animal Control officers after her neighbors allowed it roam outside without a leash, WJXT-TV, Jacksonville, Fla., reported Tuesday.
The third-grader wrote in her letter to Mayor John Peyton that although the cat was not wearing a leash, it was not "running wild."
"Other neighbors complained," Brianna wrote. "They called Animal Control and they came and said 'If you don't put your cat inside, you have to pay $250.' They said, 'No, that's crazy,' and they took the cat away. I think that is crazy and unfair."
Alicia Davey, Brianna's mother, said she felt it was important to mail the letter after she read it.
"I thought it showed such great initiative that a third-grader would take the time to take a cause to heart and do something about it," she said. "She's got such a big heart and I am so proud of her."
Peyton's office said the mayor is planning to reply to the girl's letter personally.