Peterson's mother, Sylvia, said on an episode of "The Doctors" that the incident "affected her forever."
"At the height of Mandy's career, she suffered a very serious trauma," Sylvia Peterson told the program. "She had been raped."
Her mom added, "She just felt so ashamed. She didn't want people to know."
"After that she became more defensive, less trusting," the actress' father, James Peterson, said on the program. "Some of the sparkle was gone."
The "Can't Buy Me Love" star was found dead at age 43 in her Greeley, Colorado, apartment on July 5, after a troubled adulthood which included drug problems and several arrests.
Her family, including her brother and sister, told "The Doctors" that after the rape she began using heroin and, later, meth, though she was clean, they said, for the last two years of her life.
"I think for her to carry the secret, this burden of the assault, must have just been overwhelming," sister Anne-Marie said.