Margot Kidder passed away at her home in Livingston, Montana at the age of 69 on Sunday, which was Mother's Day.
The brunette beauty from Canada was best known for playing Lois Lane in four Superman movies with Christopher Reeve in the 1970s and 80s. She also was the star of the massive 1979 horror film The Amityville Horror with James Brolin and Rod Steiger.
It is not known yet what Kidder - who once dated directors Brian De Palma and Steven Spielberg - died from, according to TMZ. Police were called to her home by an unidentified person and when officials arrived, she was 'unconscious and not breathing.'
The icon had been ill; when she called into Detroit's The Drew And Mike Show on May 9, she said she had been 'in bed with the flu.'
Her death is under investigation but foul play is not suspected.
When starring in the Superman movies, Margot was considered a pinup and often landed magazine covers, earning her a large fan base.
But all was not easy for her and for her Superman. Reeve died in 2004 after being paralysed and confined to a wheelchair due to a horse riding accident in 1995.
Margot Kidder opposite Christopher Reeve in Superman.
Margot battled bipolar disorder for years and was very open about it. Her illness got so intense she was homeless for a while in 1996 after a breakdown that led her to disappear for four days.
She was found in a backyard by a homeowner and was taken by the Los Angeles Police Department to Olive View Medical Center in a manic state. Kidder was later placed in psychiatric care.
Years later Margot said she had not had a manic episode for a long time because she had been cured by orthomolecular medicine. The movie icon went on to become an advocate for mental health awareness.
She also thrived as an actress after she got better.
In 2001, she played the abusive mother of a serial killer on Law & Order: Special Victims Unit.
Kidder appeared on Broadway in The Vagina Monologues in December 2002, and toured with the show for two years.
In 2004, Kidder briefly returned to the Superman franchise in two episodes of the television series Smallville, as Bridgette Crosby opposite her Superman co-star, Reeve.
And in 2015 Kidder won an Emmy award for Outstanding Performer in Children's Programming for her performance in RL Stine's The Haunting Hour.