McAdams was born in London, Ontario and grew up in the nearby town of St. Thomas. Her father, Lance, is a truck driver, and her mother, Sandra, is a nurse. She has a younger brother, Daniel, and a younger sister, Kayleen. She took up competitive figure skating at the age of four and acting at the age twelve at a summer theatre camp in St. Thomas, Ontario named Original Kids. When the company extended to a year-round company (and eventually relocated to London, Ontario), she was invited to continue with them.