Margaret Ruth Kidder, one of five children, was born on October 17, 1948, in Yellowknife, Northwest Territories, the daughter of Jocelyn Mary "Jill" (née Wilson), a history teacher, and Kendall Kidder, an explosives expert and engineer. Kidder was born in Yellowknife because of her father's employment, which required the family to live in remote locations. Her father subsequently served as the manager of the Yellowknife Telephone Company from 1948 to 1951. Her mother was Canadian, from British Columbia, while her father was an American, from New Mexico. She was of Welsh and English descent. She had one sister, Annie, who is a Canadian actress and executive director of the "People for Education" charity, and three brothers: John, Michael and Peter. Two of her siblings married notable Canadian personalities: Annie to actor Eric Peterson, and John to politician Elizabeth May. Kidder's niece Janet Kidder is also an actress.