Margot Kidder was born in Yellowknife, Northwest Territories, Canada, on October 17, 1948. She was one of five children of Jill and Kendall Kidder, an explosives expert. She has a sister, Annie, and three brothers, John, Michael and Peter. Kidder's niece, Janet Kidder, is also an actress.
Among her early roles was an appearance as the barmaid in Nichols, a short-lived James Garner vehicle made for American television. She also appeared in a number of low-budget Canadian movies in the early 1970s before going on to star in the Brian de Palma psychological thriller Sisters (1973) and the horror film Black Christmas (1974). A nude pictorial of Kidder, photographed by Douglas Kirkland, was published in the March 1975 issue of Playboy. The accompanying article was written by her.
Kidder is best known for her role as Lois Lane in the 1978 movie Superman and its sequels. Kidder brought more depth to the role than previous actresses, portraying Lane as an ambitious and headstrong, yet vulnerable and emotionally lonely woman trying to make it in a man's world. After she publicly expressed her disgust to the producers, Alexander Salkind and Ilya Salkind, over the firing of Richard Donner from 1980's Superman II, her role in 1983's Superman III consisted of less than 5 minutes of footage. Her role in 1987's Superman IV: The Quest For Peace was more substantial.
In addition to the Superman movies, Kidder has starred in The Amityville Horror, Willie and Phil, and The Great Waldo Pepper opposite Robert Redford. She has also made uncredited cameo appearances in Maverick and Delirious.
In 1983, Kidder produced and starred as Eliza Doolittle in a TV version of Pygmalion with Peter O'Toole. She has also done extensive stage work, including The Vagina Monologues.
In 1994, Kidder played the bartender at the "Broken Skull" tavern in Under a Killing Moon, an IBM PC adventure game.
In 2004, Kidder briefly returned to the Superman franchise in two episodes of the television program Smallville, as Dr. Bridgette Crosby, an emissary of Dr. Swann (played by her Superman co-star, Christopher Reeve).
Also that year Kidder made an appearance on a short-lived Canadian sitcom, Robson Arms, set in an apartment block in Vancouver's west end. She played a quirky neighbor of the main cast members.
In the past Kidder dated former Canadian prime minister Pierre Trudeau. She has been married and divorced three times: to American playwright Thomas McGuane (by whom she had her only child, daughter Maggie [now Maggie Kirn], in 1976); to actor John Heard; and to French film director Philippe de Broca (she suffered a miscarriage during that marriage). None of the marriages lasted longer than a year. Since her divorce from De Broca, she has said that she prefers the companionship of her dogs.
Kidder raised some hackles in the early 1990s during the first Gulf War, when she ridiculed the press and the military for not seeing the larger consequences of their actions.
Kidder became a United States citizen on August 17, 2005, in Butte, Montana; she lives in nearby Livingston. She said the reason for her decision to become an American citizen is to participate in the voting process, to continue her protests against U.S. intervention in Iraq, and at the same time to be free of worries about being deported.
Kidder has bipolar disorder which led to a widely publicized manic breakdown in 1996. She is an advocate of orthomolecular medicine as a treatment for the disorder.