He served on the north-west frontier of India from 1935 to 1938, and during the Second World War saw active service in the Sudan, north Africa, Italy, France, and Germany.

[1] After service with the provost marshal's branch of the War Office, in early 1950 Major Muir was posted to the Argylls' 1st battalion in Hong Kong.

In August 1950 the battalion moved to Korea, among the first British troops to join the United Nations forces in the Korean War.

His citation noted that: "The effect of [Muir's] splendid leadership on the men was nothing short of amazing and it was entirely due to his magnificent courage and example and the spirit which he imbued in those about him that all wounded were evacuated from the hill, and, as was subsequently discovered, very heavy casualties inflicted on the enemy in the defence of the crest.

[1] He was awarded the following medals,[4] which are displayed at the Argyll and Sutherland Highlanders Museum, Stirling Castle, Scotland.