He served one term as the national President of the Canadian Bar Association. Senator Thérèse Casgrain, who campaigned for women"s equality and their right to vote, was a distant relative by marriage. Casgrain was educated at a boarding school, Saint-Jean-Berchmans at Québec, at the Séminaire de Québec, at Saint Procopius College (now the Benedictine University) in Chicago, Illinois, at the Collège de Sainte-Anne-de-la-Pocatière and at the Université Laval.
He volunteered for the Canadian Army during World War I, as an officer in the 1st Canadian Tank Battalion.
He was called to the Bar of Quebec in 1920 and practised in Rimouski from 1920 to 1974, and in Montreal from 1974 onwards. From 1920 to 1936, he was the Crown prosecutor for the Rimouski district, being named King"s Counsel ("conseillier du roi") in 1930.
From 1942 to 1944, he served as Minister without portfolio in the Cabinet of Premier Adélard Godbout. Casgrain was active in the bar of Quebec and the Canadian Bar Association throughout his career.
In 1967-1968, he served as national President of the Canadian Bar Association.
He died in Montreal in 1981 at age 83.
He was a member of the Legislative Assembly of Quebec for Gaspé-Nord from 1939 to 1944.