Marie Thérèse Forget was born in Montreal, Quebec, into a wealthy and politically connected French-Canadian family.
Thérèse Casgrain (née Forget) was born on July 10, 1896, in Montreal, Quebec, the daughter of Sir Rodolphe Forget, a prominent financier and Member of Parliament, and Blanche MacDonald.
Growing up in an affluent household gave her access to education and social connections that were unavailable to most women of her era, yet as a woman in early twentieth-century Quebec she was still barred from voting, practicing law, or holding public office. This tension between privilege and structural exclusion would shape her lifelong commitment to social justice.
She was educated privately and at convent schools, receiving the polished, socially oriented education typical for young women of her class, rather than the rigorous academic training given to her brothers. This early experience of gendered inequality within her own family became a formative influence, and she later credited her awareness of women's unequal treatment to observations made during her childhood and early married life.
Her birth into the Forget family, one of the most influential French-Canadian families in Montreal's business and political elite, provided her with the resources, connections, and platform she would later use to become one of Canada's foremost campaigners for women's suffrage, social welfare, and human rights.