Born in Brussels, her father Paul Janson was leading member of the progressive wing of the Belgian liberal movement and founder of the Fédération progressiste.
The couple had four children, of whom Paul-Henri Spaak, later Belgian Prime Minister like his uncle Paul-Émile, was the most famous.
During the First World War Marie was active in social work and this led her to join the Socialist Party.
She was elected to the municipal council of Saint-Gilles in 1921, and the same year was selected by the party's executive, led by Émile Vandervelde, to serve in the Belgian Senate.
Her granddaughter Antoinette Spaak followed in her footsteps, achieving another first: first woman to lead a Belgian political party, the Democratic Front of Francophones.