Mason-MacFarlane joined the Royal Artillery in 1909 and served in the First World War on the Western Front and in Mesopotamia. He was awarded the Military Cross and the Distinguished Service Order. He served as Britain"s military attaché to Berlin prior to the Second World War (and proposed the assassination of Hitler, an offer turned down by his superiors) as well as to Hungary, Austria, Switzerland and Denmark.
He was Director of Military Intelligence with the British Expeditionary Force (BEF) in 1939–1940 and during the retreat to Dunkirk was operational commander of "Mac Force," an improvised formation covering the British right flank.
From July 1940 to March 1941 Mason-Macfarlane was Second in Command of Gibraltar City and Garrison. This position allowed him to head the Joint Intelligence Centre.
He was the head of a joint group of Army, Navy and Royal Air Force personnel whose role it would be to support General Franco if Spain were to be invaded by Germany. They were to assist the Spanish defence and, if the Spanish did not resist, then they were to create maximum damage.
He was next Head of the British Military Mission in Moscow, from 1941 until 1942.
He then became General Officer Commanding 44th Division. He was Governor of Gibraltar from 31 May 1942 to 14 February 1944, and witnessed the air crash there on 4 July 1943 which took the life of his friend the Polish Prime Minister Władysław Sikorski. He later served as Chief Commissioner of the Allied Control Commission for Italy in 1944, effectively head of the interim post-war government.
He left parliament for ill-health reasons on 22 October 1946 by accepting the Chancellor of the Exchequer"s appointment as Steward and Bailiff of the Manor of Northstead.
lieutenant was reported in Time magazine on 24 August 1953 that "one of Britain"s ablest soldier-administrators" had died of arthritis and complications from a broken legal Mason-Macfarlane"s papers and correspondence are archived in the Imperial War Museum"s Department of Documents.
The citation reads as follows:
While he was engaged on a reconnaissance another officer who was with him was severely wounded by a sniper. He removed him to a place of safety and also brought in a stretcher-bearer who was wounded by the same sniper.
He then completed his reconnaissance and returned with valuable and accurate information.