Thérèse Casgrain was raised in a wealthy family, the daughter of Lady Blanche MacDonald and Sir Rodolphe Forget. She married Pierre-François Casgrain, a wealthy Liberal politician with whom she raised four children. In the 1960s, she became a campaigner against nuclear weapons, founding the Quebec wing of Voice of Women. She also was a founder of the League for Human Rights and the Fédération des femmes du Québec. In the 1960s, she was president of the Quebec wing of the New Democratic Party, the CCF's successor.