Having joined the chambers of Montague Berryman KC at 12 King's Bench Walk, he practised common law on the Western Circuit.
[1] Having joined the Royal Naval Volunteer Reserve in 1936, he served in the battleship HMS Barham, then in minesweepers for most of the Second World War.
In 1940, abroad HMS Ross, he participated in Operation Dynamo, the evacuation of the British Expeditionary Force from Dunkirk.
In 1971, following in the steps of his father, Croom-Johnson was appointed a Justice of the High Court, assigned to the Queen's Bench Division and receiving the customary knighthood.
In 1977 he presided over the trial of the IRA leader Vincent Donnelly for carrying out the West Ham station attack.