He was named professor of law at Université Laval two years later. In 1885, he was named to the legal team representing the Crown in the trial of Louis Riel. Although the crown was represented by a large team including George Burbidge, Christopher Robinson, Britton Bath Osler and others, Casgrain was the only French-Canadian in the group.
Pro-Riel sentiment in the province of Quebec was so strong that he was burned in effigy during at least one demonstration.
However, this did not prevent him from being elected in the Legislative Assembly of Quebec. He was elected for Quebec County in 1886 and for Montmorency in 1892.
He was attorney general in the provincial cabinet from December 1891 to November 1892 and again from December 1892 to May 1896. He served as Postmaster General from October 1914 until his death.
He represented Montmorency as a Conservative Member of Parliament in the Canadian House of Commons from 1896 to 1904 and Quebec County from 1914 until his death from pneumonia at Ottawa in 1916.