- Kidder, who played Lois Lane in Superman, was found dead by a friend on May 13
- Daughter Maggie McGuane said it was a 'big relief' for coroner to release ruling
- Kidder struggled with mental illness much of life, made worse by 1990 car crash
- In final months her home was taken over by meth heads she was trying to 'fix'
- Kidder and Christopher Reeve starred in four of the films between 1978 and 1987
Superman actress Margot Kidder took her own life with a 'self-inflicted drug and alcohol overdose', a coroner ruled on Wednesday.
Kidder, who played Lois Lane opposite Christopher Reeve's Superman in her most famous role, was found by a friend in her Montana home on May 13.
She struggled with mental illness throughout much of her life and spent her final months at home surrounded by meth addicts who she was trying to 'fix', according to friends.
Park County coroner Richard Wood said in a statement she 'died as a result of a self-inflicted drug and alcohol overdose' and that no further details would be released.
Maggie McGuane, Kidder's daughter by her ex-husband Thomas McGuane, said she knew her mother had taken her own life the moment officials took her to Kidder's home in Livingston, a small town near Yellowstone National Park.
'It's a big relief that the truth is out there,' she said. 'It's important to be open and honest so there's not a cloud of shame in dealing with this.'
When starring in the Superman movies, Margot was considered a pinup and often landed magazine covers, earning her a large fan base.
Kidder battled bipolar disorder for years and was very open about it.
It was made worse by a 1990 car accident that left her in debt and led to her using a wheelchair for almost two years.
Six years later, she was homeless for a while after a breakdown that led her to disappear for four days.
She was found in a backyard by a homeowner and was taken by LAPD to Olive View Medical Center in a manic state. She was later placed in psychiatric care.
Years later Kidder said she had not had a manic episode for a long time because she had been cured by orthomolecular medicine.
Her co-star Reeve died in 2004 after being paralyzed and confined to a wheelchair due to a horse riding accident in 1995.
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.
Her last project was the film The Neighborhood with Danny Aiello, which came out in 2017.
Kidder's death is one of several high-profile suicides this year that include celebrity chef Anthony Bourdain and fashion designer Kate Spade.
McGuane noted that Montana has one of the highest suicide rates in the nation and she urged people with mental illness to seek help.
'It's a very unique sort of grief and pain,' McGuane said. 'Knowing how many families in this state go through this, I wish that I could reach out to each one of them.'
Kidder got her start in Hollywood with the 1968 film The Best Damn Fiddler From Calabogie To Kalada.
The next year she was cast opposite Beau Bridges in Gaily, Gaily, but it did little for her profile.
Then the star popped up in TV shows such as Wojeck, Adventures in Rainbow Country, McQueen, and Nichols.
She struggled for years in film in LA and on stage in New York City until Brian De Palma cast her in the 1973 film Sisters in which she played conjoined twins. The two had a brief romance on the movie.
Next came the Black Christmas, The Gravy Train and A Quiet Day in Belfast, all of which came out in 1974.
The next year she worked with Robert Redford in The Great Waldo Pepper then with Peter Fonda in 92 In The Shade, making her a strong leading lady.
In 1976 she took time off to welcome her daughter, Maggie - now 41-years-old - her first husband, American novelist Thomas McGuane.
Two years later she was cast as Lois Lane in Superman, which was a smash success. Three sequels followed.
Between Superman films she made 1979's blockbuster The Amityville Horror, 1981's Heartaches, 1981's Some Kind Of Hero, 1983's Trenchcoat, 1984's The Glitter Dome, and 1985's Little Treasure.
She appeared in stage versions of Bus Stop and Pygmalion as well.
The beauty was married and divorced three times.
Her first husband was McGuane. Next came actor John Heard. They wed in 1979 but split six days later. She was married to French film director Philippe de Broca from 1983 to 1984.
The star also dated former Canadian prime minister Pierre Trudeau, directors Brian De Palma, writer/director Tom Mankiewicz and Steven Spielberg, as well as stand-up comedian Richard Pryor.
Kidder, a native of Yellowknife, Canada, was also a political activist who was arrested in 2011 in a Washington, D.C., protest over the proposed Keystone XL pipeline from Canada's oil sands.
Her final years were troubled by conflicts with people who were down on their luck that she took into her home.
The actress-cum-activist's home in Livingston, Montana, was taken over by meth-heads who she was trying to 'fix', close friends told Dailymail.com in May.
Between August 2016 and her death in May, authorities were called to her house 40 times on reports of people trespassing, theft and other disturbances, according to police logs released under a public-records request.
The calls include responses by ambulances five times in seven months, including at the time of her death.
Drug addicts ended up cooking methamphetamine in her basement and stealing her valuables, they added.
'Margie was a real bad judge of people,' environmental activist Louisa Willox said, using the name that the Superman star was universally known by around the town, which has attracted dozens of counter-culture characters over the years.
'Towards the end I went round to help her with her medications and I couldn't read the instructions on the bottle because the ink had run.
'She told me that was because she had to hide the pills in her bra to stop these guys stealing them.'
Joan Kesich, a longtime friend who found Kidder's body, said Kidder was fearless and always spoke the truth, regardless of the consequences.
'In her last months, she was herself - same kind of love, same kind of energy,' Kesich said.
'The challenges that she had were very public. I want what I know about her to be out there because it was glorious. She was really a blazing energy.'
At the time of the Kidder's death, her manager, Camilla Fluxman Pines, said she died peacefully in her sleep.