Gagnon was born into and raised in a Roman Catholic family. She was the second of five children of farmer Télesphore Gagnon and his first wife Marie-Anne Caron, whom he had married in September 1906. They lived in Fortierville, Quebec, a small village on the south shore of the St. Lawrence River, a hundred kilometers southwest of Quebec City. The Gagnons' first child, Marie-Jeanne, was born in August 1907. Aurore's birth was quickly followed by that of Lucina, then Georges-Étienne in 1910 and Joseph in 1915.