Kidder died on May 13, 2018, at her house in Livingston, Montana, at the age of 69. She was found unresponsive by a friend. The cause of death was initially not released; her agent stated that "she passed away peacefully in her sleep," while, in the days following her death, her friend Jeffrey St. Clair wrote: "I've been struggling all week with the image of Margie lying helplessly on the floor of her house." On August 8, 2018, it was reported that Kidder's death had been ruled a suicide by overdose. The Park County, Montana, coroner said her death was "a result of a self-inflicted drug and alcohol overdose." Kidder had been scheduled to appear at a Comic Con event in London, UK, the following month.