"Cathriona White, 30, was pronounced dead by paramedics Monday evening," Fred Corral, a spokesman at the Los Angeles County coroner's office, told AFP.
He said White's death was being treated as a suicide.
Local news reports said the couple, who have been dating on and off since 2012, broke up last Thursday.
White sent a cryptic message on her Twitter account that day that read: "Signing off Twitter, I hope I have been a light to my nearest and dearest."
More From This Section
"She was a truly kind and delicate Irish flower, too sensitive for this soil, to whom loving and being loved was all that sparkled," the 53-year-old actor added.
"My heart goes out to her family and friends and to everyone who loved and cared about her. We have all been hit with a lightning bolt."
White was found at her home Monday evening by a friend who called emergency services. She had reportedly left behind a note.