Franzen-Davis Funeral Home in Livingston, Montana, posted a notice on its website saying Kidder died Sunday at her home there.
"The actress and activist passed away on Sun., May 13, 2018 at her home", the funeral home said. She became a household name after Superman was a massive box office hit in 1978.
Margot was most famous for playing Superman's leading lady, Lois Lane, in the 1978 movie alongside Christopher Reeve, as well as in the following three sequels.
She also made a cameo appearance in the 1987 film about the DC Comics superhero titled "Superman IV: The Quest for Peace".More news: Iran nuclear deal: The EU's billion-dollar deals at risk
It is not known yet what Kidder - who once dated directors Brian De Palma and Steven Spielberg - died from, according to TMZ. "These are hard words to write, so simply - you will be so very missed, my dear Margot", he told SyFy.
She said she and Reeve, who was paralyzed in 1995 by a horsing accident, were like brother and sister, both in their affection and animosity for each other. "Then we'd play chess or something because we were also really good friends".
The Reeve Family and Foundation said it was "deeply saddened" by Kidder's death.
Chief Strategist for Barack Obama's presidential campaigns, David Axelrod, wrote, "Margot Kidder should be remembered as much for courageously, candidly discussing her battles with mental illness as for any screen role". Off-screen she was one of the kindest, sweetest, most caring woman I've ever known.More news: HSBC claims first commercially viable trade finance deal on blockchain
That struggle became public in 1996 when she was found dazed and filthy in a yard not far from the studio where she once filmed parts of Superman.
Hollywood soon beckoned and a move to Los Angeles led to roles on TV shows including "McQueen", "Mod Squad" and "Nichols".
"The reality of my life has been grand and wonderful, punctuated by these odd blips and burps of madness", she told People shortly after.
Kidder was born in Yellowknife, Canada and graduated from a Toronto boarding school before pursuing acting. She is survived by her daughter Maggie McGuane.More news: NASA is sending a tiny robot helicopter to Mars
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