Investigators with the Society for the Prevention of Cruelty to Animals said 14 cats were in the house, living in inhumane conditions, WCAU-TV reported. The agency also removed the carcass of a dead cat.
The 81-year-old owner of the house was arrested.
George Bengal, director of humane enforcement for the SPCA in Philadelphia, said the organization had received tips for years about the animals in the mansion but had been unable to get inside to check them out. But he appeared to have sympathy for the woman involved.
"I think the lady is a rescue lady and has been for years," Bengal said. "This just turned into a hoarder situation where she didn't have the resources to care for these animals."
The woman had been living in the house since 1973. Grace Kelly's mother lived there until her death in 1970.
Grace Kelly was born in Philadelphia where her father, Jack Kelly, was a bricklayer turned Olympic sculling champion and construction mogul. She became one of the top Hollywood stars of the early 1950s before marrying Prince Rainier in 1956.
The princess died in 1982 in a car crash in Monaco.