Baptized Mary Ellen Gueren, the ancestors of Mary Ellen Gueren's father originated in France and immegrated to Ireland in the 1600s. In the 1830's the family came to Lower Canada and settled in Montreal.

At the age of 17 Mary Ellen would be described as having become an accomplished poet as well as author of historical sketches of prominent women in New France and it is during this time we speculate she adopted the name Bellelle Guerin.

By 1917 Bellelle became president of the Catholic Women's Club, formerly the Ladies of Loyola Club. That June she wrote to Montreal's Roman Catholic archbishop, Paul Bruchesi, requesting his blessing for the formation of another Catholic women's group, and by November she called a meeting at which the Montreal branch of the CWL was founded, with herself as first president.

By 1920 the Montreal branch grew to 440 members and and by June, at Bellelle's instigation, a meeting was called to unify the branches of the CWL, she was elected first dominion president of the national federation, the Catholic Women's League of Canada.

The following year at the first national convention, held in Toronto, Ontario, she expressed her belief in the future of the organization. "we may be said to be laying the cornerstone of an edifice that will arise fair and beautiful, strong and proud before the eyes of the world."

Bellelle's role in the CWL was recognized in 1922 at a meeting of the International Union of Catholic Women's Leagues in Rome, when she became the first woman in Canada to receive the Pro Ecclesia et Pontifice from the papacy.