During the First World War, he served with the machine gun battalion of the 51st (Highland) Division in France, winning the Military Cross in 1918.

After serving with the Machine Guns Corps in France, Belgium and Germany during the First World War, for which he was awarded the Military Cross and reached the rank of Lieutenant, MacDermott was called to the Bar of Ireland in 1921.

[citation needed] In 1936 he was made a King's Counsel, and two years later he was elected to the Northern Ireland House of Commons as an Ulster Unionist member for Queen's University.

[citation needed] In 1944 he resigned his parliamentary seat on appointment as a High Court Judge for Northern Ireland, and three years later, on 23 April 1947 was made a Lord of Appeal in Ordinary, becoming a life peer as Baron MacDermott, of Belmont in the City of Belfast.

In 1977, aged over eighty, Lord MacDermott offered to redeliver a lecture at the Ulster College, which had been interrupted by a bomb meant for him and which had severely wounded him.

Their son, Sir John MacDermott, was also sworn into the British Privy Council in 1987, as a Lord Justice of Appeal in Northern Ireland.