Duncan joined the NWMP on August 31, 1903. At the time of his engagement he was 6’ 3½” tall, 169 pounds, with brown hair and blue eyes. He married Miss L. McBeth, the daughter of the Hudson Bay Company manager at Fort Pelly in 1907. His wife died in 1909 leaving him with one child. He later re-married. He was discharged from the NWMP — his time expired — August 30, 1909. For some nine years following this he is believed to have farmed in the Edmonton district.
On August 21, 1918 Duncan joined the Alberta Provincial Police as a First Class Constable. He served at Lloydminster, Wainwright, and in the Crow’s Nest Pass area. He was promoted to Corporal in 1920 and to Sergeant in 1923. In July 1928 he was made an Inspector and transferred to Edmonton to take charge of the Criminal Investigation Branch. On July 1, 1931 he was transferred to Peace River, Alberta. He was the Officer Commanding the Peace River District until 1932. When the R.C.M.P took over the duties of the Alberta Provincial Police in 1932, Assistant Commissioner Duncan was appointed an Inspector with the Force. He remained in the Province of Alberta until 1936 when he went on the Eastern Arctic Patrol as Inspecting Officer. On his return he was posted to command the Weyburn, Sask. Sub-Division.
From that point he was transferred to Regina and, after spending a short time there, was ordered to take command of the Sydney Sub-Division in Nova Scotia.
His promotion to the rank of Superintendent in 1938 brought with it further responsibilities and he was appointed Assistant Director of criminal Investigation at Ottawa, where he remained for a number of years.
In 1943 he was posted to Washington as Liaison Officer with the United States Federal Bureau of Investigation. The following year he was appointed Assistant Commissioner in the RCMP. On November 1, 1946, after completing thirty-four years police service he was granted retirement to pension.