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.
Casgrain led the women's suffrage movement in Quebec prior to World War I. She founded the Provincial Franchise Committee in 1921 and campaigned for women's rights and for the right to vote in Quebec elections, a right that was not won until 1940. From 1928 to 1942, she was the leader of the League for Women's Rights. In the 1930s, she hosted a popular radio show Fémina. In the 1942 federal by-election, she stood as an "Independent Liberal" candidate in the Charlevoix-Saguenay riding, the same seat formerly held both by her father and by her husband.
Following World War II, she left the Liberal Party and joined the social democratic Co-operative Commonwealth Federation (CCF). In 1948, she became one of the federal vice presidents of the CCF. She led the Quebec wing of the party, the Parti social démocratique du Québec, from 1951 to 1957 through three provincial elections using her position as a platform to campaign against the government of Maurice Duplessis. She was therefore the first female leader of a political party in Canada.
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.
In 1967, she was made an Officer of the Order of Canada, and in 1974, she was promoted to Companion.
In recognition of her achievements, Prime Minister Pierre Trudeau appointed Casgrain to the Canadian Senate in 1970, where she sat as an independent for nine months before reaching the mandatory retirement age of 75.
Thérèse Casgrain's body is interred in the Cimetière Notre-Dame-des-Neiges in Montreal.