According to TMZ, Kidder's cause of death is currently unknown, but foul play has been ruled out. Park County Attorney Bruce Becker said that someone called police to report Kidder was unconscious and not breathing in her home. Police responded and found her dead.
Kidder's career took off when she starred opposite Christopher Reeve in the 1978 "Superman" movie as well as the film's three sequels. Despite a very public battle with bipolar disorder, Kidder continued to take on small acting roles in TV shows and movies up until this year. She also had a part in "The Vagina Monologues" on Broadway.
During her struggle with bipolar disorder, Kidder was briefly homeless in 1996. She later became an advocate for mental health awareness.
"On screen, she was magic. Off screen, she was one of the kindest, sweetest, most caring [women] I've ever known," Mark Hammill tweeted. "I'll miss you, [Margot Kidder]. Your legacy will live on forever." Elijah Wood also expressed his sadness on Twitter, simply writing, "Oh no, Margot Kidder."
See how other stars and fans are mourning the loss below:
On-screen she was magic. Off-screen she was one of the kindest, sweetest, most caring woman I've ever known. I'll miss you #MargoKidder. Your legacy will live on forever. pic.twitter.com/UBlbszEIhb
#MargotKidder was incredible in the Superman movies. The Lois Lane she created had so much strength, humor and vulnerability that she became as much a part of what made the movie work as the wonderful Chris Reeve, Gene Hackman and others. #ripmargotkidder
Rest In Peace Dear #MargotKidder ❤️ I was such a fan of yours from the first moment I saw you on the screen as Lois Lane in Superman! I will treasure the moment that I met you forever! Sweet dreams Margot🌤
Margot Kidder should be remembered as much for courageously, candidly discussing her battles with mental illness as for any screen role. There are no Supermen— or women. It is not a character deficiency to acknowledge such struggles and get the help and treatment you need! RIP pic.twitter.com/ZD6RB3cfzU
Sad to hear of the passing of Margot Kidder. A spark of vivacious life in all of her films. I loved her in cult horrors 'Sisters' and 'Black Christmas'. And of course she remains the best Lois Lane in the magical 'Superman' & 'Superman II'. RIP to a unique screen presence. pic.twitter.com/bQEVv7PSa3
RIP Margot Kidder. An actor who imbued all her roles with grit, strength and smarts. I admired her very much and she's gone way too soon. I chatted with her a couple of times at conventions. Happy I told her what she meant to us