At the time of her death, Kidder's manager, Camilla Fluxman Pines told the press that Kidder died peacefully in her sleep at her home in Livingston, a small town near Yellowstone National Park.
Canadian-born Kidder had been a mental health advocate during her life with her own struggles well-documented in the media.
The AP also reports that the Superman actress was living in a state of chaos in her final years, as her attempts to provide refuge for those in need resulted in authorities being called to her home 40 times between August 2016 and her death. She also noted Montana's high suicide rate and the empathy she feels for the people there going through the same thing.
She went on to receive a Daytime Emmy Award as outstanding performer in a kids' series in 2015 for that role.
"It's important to be open and honest so there's not a cloud of shame in dealing with this", she continued.
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In a joint statement, Kidder's family and the Park County Coroner's Office urged: '...those suffering from mental illnesses, addiction and/or suicidal thoughts to seek appropriate counselling and treatment'.
"It's a big relief that the truth is out there", the daughter, Maggie McGuane, said.
Most recently, Margot appeared in The Neighbourhood, which came out earlier this year.
"In her last months, she was herself - same kind of love, same kind of energy", Kesich said.
Both Kidder and co-star Christopher Reeve, who played Superman, were relative unknowns when they got their leading parts, and neither saw many major roles afterwards.