The actress died at her home in Livingston, Montana a funeral home has confirmed.
Margot battled bipolar disorder for years and was very open about it. It is unclear at the moment how she died, but how she lived was as an outspoken and larger than life figure who found a unique perch in 1970s and '80s pop culture.
In this way, she turned that dark chapter into something positive. She later became a major advocate in the mental illness community. Personally, Superman was the first superhero movie I ever saw, and I distinctly remember thinking I'd love to have a girlfriend like Lois one day.
Statford, Ont. -based comic creator Scott Chantler said Kidder's portrayal of Lane transformed the character from a "damsel in distress" awaiting rescue, to a "force of nature" whose brashness made her a crackling counterpart to Superman's nebbish alter-ego, Clark Kent. "No one noticed the difference one from the other - it worked".
It was in 1978 that Kidder got her big break in Richard Donner's Superman.
Kidder was taken by the Los Angeles police to a hospital and then was placed in psychiatric care.
"She was kind of an indomitable person", she said in a phone interview.
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It is not yet known what Kidder died from.
However, her career came to a halt after she was seriously injured in a vehicle crash, leaving her unable to work for two years.
As news broke of Margot's death tonight, her Lois Lane Superman co-star Teri Hatcher took to Twitter to pen a heartfelt message. It was at her infamous beach house north of Malibu - shared with fellow actor Jennifer Salt - that a number of crucial meetings occurred between some of the figures, mostly male, who remade Hollywood film-making in their own movie-crazy image during what is now known as the Hollywood Renaissance. Beyond that, though, she mostly stayed out of the headlines notwithstanding her political activism and the fact that she was, for a time, linked to a romance with former Canadian Prime Minister Pierre Trudeau.
But following her 1996 breakdown, she began to recover her mental health. In 2007 that she hadn't had a manic episode in 11 years. "She was a source of human strength in a film about superhuman strength". She also appears alongside Christopher Reeve in one of the most famous scenes in cinema history.
Some thought this episode signaled the end of Kidder's career, that she would retreat from the public eye. In 2015, Margot Kidder won an Emmy Award for performance on the children's TV series, R.L. Stine's The Haunting Hour.
Kidder became an American citizen in 2005.