Kidder lived in Livingston, Montana, according to the Franzen-Davis Funeral Home & Crematory. Funeral arrangements were pending Monday.
Her cause of death was not immediately known, according to TMZ. The celebrity news site was the first to report on Kidder’s death Monday.
Kidder rose to fame in 1978 while starring in the first of a series of Superman films opposite Christopher Reeve. She appeared in three subsequent installments of the series from 1980 to 1987.
The Canadian-born actress appeared in dozens of films during her career, including 1974's "Black Christmas" and 1979's "The Amityville Horror."
Despite her success, Kidder was homeless for a brief time in the 1990s as she dealt with mental health issues. After a highly publicized breakdown in 1996, she told PEOPLE magazine that she had been diagnosed with manic depression.
“It’s very hard to convince a manic person that there is anything wrong with them,” Kidder told the magazine at the time. “You have no desire to sleep. You are full of ideas.”
Kidder was also an outspoken activist who supported liberal causes. According to The Guardian, she was an active member of the “peace movement” for years. She was arrested in 2011 during a protest in Washington of an oil pipeline, CBC News reported.
Fans and friends took to social media to mourn Kidder:
So saddened by the news that dear #MargotKidder has died peacefully in her sleep yesterday. We continued to have fun together over the last 40 years and she was a joy to be around. pic.twitter.com/16ARRj6Hy6