Marie Lacoste married a lawyer, Henry Gérin-Lajoie, on the condition that he give her the freedom to continue her campaign for women"s rights. The couple raised four children. In addition to her campaign for more legal rights for women, Gérin-Lajoie also played a part in arguing for French-language university education for the women of Quebec.
Partly in response to her actions, the Quebec Catholic clergy agreed to open the first francophone women"s college, in 1908.
In 1922, Gérin-Lajoie led a protest for women"s suffrage in Quebec. Quebec was the last Canadian province to grant the vote to women, in 1940.
She was the author of two legal works: Traité de droit usuel, in 1902 and Louisiana femme et le code civil, in 1929. In these books, Gérin-Lajoie argued against the subordinate legal position of married women.
In this time, women had no control over their own financial assets and no legal input into the financial affairs of their families.
She wanted to grant more rights to married and separated women so they could control their own property, and act as legal guardians to minors. In 1929, Gérin-Lajoie testified on women"s rights before the Dorion Commission. In 1931, the Quebec Civil Code was changed to reflect the changes Gérin-Lajoie had been arguing foreign
She was designated a Persons of National Historic Significance by the Historic Sites and Monuments Board of Canada.