McAdams was born in London, Ontario and grew up in nearby 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 age thirteen 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. She attended the Myrtle Street Public School. and the publicly-funded secondary school Central Elgin Collegiate Institute in St. Thomas from grade nine to OAC and starred in the Award-winning student production I Live in a Little Town. Later she graduated from York University in Toronto with Honours and a BFA degree in Theatre. During her fourth year at York, she played a child in The Piper – a workshop led by a creative team from Toronto's Necessary Angel Theatre Company. She also attended