Born in 1896, MacDermott was educated at Campbell College, Belfast, and the Queen's University of Belfast. 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 Irish bar in 1921. Eight years later he was appointed to determine industrial assurance disputes in Northern Ireland, and in 1931 he became a lecturer in Jurisprudence at Queen's University, teaching for four years. 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.
In 1940 MacDermott was appointed Minister of Public Security in the Northern Ireland Government, and the following year became the Province's Attorney General. In 1944 he resigned his parliamentary seat on appointment as a High Court Judge for Northern Ireland, and three years later was made a Lord of Appeal in Ordinary, effectively becoming a life peer as Baron MacDermott, of Belmont in the City of Belfast.
In 1977, aged over eighty, 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.
Having been made a Northern Ireland Privy Counsellor seven years earlier, Lord MacDermott was admitted to the British Privy Council in 1947. Four years later he was appointed Lord Chief Justice of Northern Ireland, a post he held for twenty years. He was also Pro-Chancellor of his alma mater until 1969, and in 1958 chaired the commission on the Isle of Man Constitution. He died in 1979.