He served with distinction in the British Army during the Second World War and was awarded the MC in 1944. In 1948 he was called to the Northern Ireland Bar and became a High Court judge on 8 March 1968. In 1975 McGonigal was appointed a Lord Justice of Appeal. He was knighted on 1 July 1975. He died in 1979, aged 62.
Justice McGonigal is mentioned in Tony Geraghty's The Irish War: the hidden conflict between the IRA and British Intelligence as having been "forced to carry a gun under his robe" due to terrorism in Northern Ireland, which would claim the lives of at least five judges or justices in Northern Ireland.