Santa Clarita station deputies responded around 10 p.m. to a medical rescue call in the 25000 block of Oak Meadows Drive, according to the Los Angeles County Sheriff's Department.
The woman, later identified by authorities as Josie Lynn Harris, was found unresponsive in a gated community, and firefighter-paramedics pronounced her dead at the scene. The 40-year-old is the mother of three of Mayweather's children, according to Los Angeles County Sheriff Alex Villanueva.
"It was learned the woman was discovered inside a vehicle parked in a driveway of a residence where it is believed she lived," the Sheriff's Department said in a statement.
The cause of death is under investigation. Villanueva says detectives are looking for witnesses. There were no obvious signs of violence and she was apparently alone when she died.
Harris' relationship with Mayweather had a history of domestic abuse. Mayweather served two months in jail for battering her in 2010.
She had been working on a book with Jann Robbins, a writer and widow of famed novelist Harold Robbins.
"I am brokenhearted actually, because I will miss her. I miss her bright light she always brought into the room," Jann Robbins said.
Because of the children, Robbins says Harris had a back-and-forth relationship with Mayweather.
Anyone with information about the case is asked to call Crime Stoppers at (800) 222-TIPS (8477).