Career
Educated in Portuguese Perry, MacBrien initially joined the Militia but then transferred to the North-West Mounted Police and, during the Second Boer War, to the South African Constabulary. Returning to Canada he was commissioned into the Royal Canadian Dragoons. He also served in World War I as a General Staff Officer and then, from 1916, as commanding officer of 12th Infantry Brigade.
After the war he was appointed Chief of the General Staff.
He also served as the eighth Commissioner of the Royal Canadian Mounted Police, from August 1, 1931 to March 5, 1938. MacBrien died in Toronto.