Born in Montreal, Quebec, his father was a physician. Following the death of his mother when he was three years old, he was raised by his grandmother. Casgrain graduated in law from Université Laval and practiced in Montreal where he worked as an organizer for the Liberal Party of Canada and the Quebec Liberal Party.
From 1921 to 1925, Casgrain was the parliamentary whip of the Quebec Liberal caucus, and from 1926 to 1936, he was the Chief Whip of the Liberal caucus.
1The department was eliminated in 1993 when the government was reorganized. The position of Secretary of State for Canada was not legally eliminated until 1996 when its remaining responsibilities were assigned to other cabinet positions and departments, particularly the newly created position of Minister of Canadian Heritage.