Kidder suffered with mental health issues from a young age, which stemmed from undiagnosed bipolar disorder. "I knew I was different, had these mind flights that other people didn't seem to have," she recalled. At age 14, she attempted suicide by swallowing a bottle of codeine capsules after her then-boyfriend broke up with her. Kidder found an outlet in acting as she felt she could "let my real self out... and no one would know it was me." "Nobody ever encouraged me to be an actress," she recalled. "It was taken as a joke ... As a teenager, I envisioned myself in every book I read. I wanted to be Henry Miller and Thomas Wolfe. I wanted to eat everything on the world's platter, but my eyes were bigger than my stomach." She attended multiple schools during her youth through her family's relocations, eventually graduating from Havergal College, a boarding school in Toronto, in 1966. In 1966, she found herself pregnant by her boyfriend who arranged to have an illegal abortion. The abortionist was located in a hotel room and filled Kidder's uterus with Lysol to terminate the pregnancy. After graduating from Havergal, Kidder relocated to Vancouver to attend the University of British Columbia, but dropped out after one year. She returned to Toronto, where she found work as a model.