He joined the 89th Temiscouata and Rimouski Battalion of Infantry at the age of 16. His military career continued during his studies, he was promoted to the rank of lieutenant while a student. In 1899, he succeeded his father as surgeon to the 89th Battalion but, a few months later, joined the Second (Special Service) Battalion of The Royal Canadian Regiment with the rank of major and saw service in the South African War.[2] For his service in the war, he was appointed a Companion of the Distinguished Service Order (DSO) in the October 1902 South African Honours list.[3]
After a year he returned to Rimouski before becoming an adjutant in the Royal Canadian Army Medical Corps. In 1902, he was part of the Canadian Coronation Contingent for King Edward VII's coronation and travelled to the United Kingdom as the Surgeon-Lieutenant of The Royal Canadian Regiment. In 1903, he was promoted to the rank of colonel and became Director-General of the service.[2]
Fiset was promoted to the rank of Surgeon General (Major General) at the beginning of World War I.[4][5][6][7][8] He was knighted for his service in the war while the French government named him a Commandeur of the Legion of Honour.[2]
On December 30, 1939, Fiset became Lieutenant-Governor of Quebec. On two occasions during the war he opened the legislative assembly wearing his major general's uniform in order to express support for the war effort. He remained in office until October 1, 1950.[2]