Sir Moir Mackenzie joined the FBI in June 1918, and was Deputy Director from 1946 to 1951, when he retired after 33 years' service. With Charles Ramsden, he played a notable part in creating the FBI Overseas organisation. he took particular interest in Commonwealth affairs, and had close contact with fellow Scots in Canada. One of his last activities was to help promote the formation of the Canadian Association of British Manufacturers and Agencies. He was acting Director of the FBI during the war and dies in January 1963, aged 76.
Only a small number of his files have survived.