 Stephenson was born in Aurora, Ontario, the daughter of Clara Mildred (Draper) and Carl Melvin Stephenson.She graduated from Earl Haig Secondary School in Willowdale, Ontario in 1941 and was the only female in her class to go on to university.She entered University of Toronto Medical School at the age of 17, a year younger than what was then the minimum age for admission, after persuading the dean to waive both the rule and the $680 tuition fee.