McLachlan (born 25th September 1908) was educated at the City of London School and Magdalen College, Oxford, where he gained first class honours in PPE. He was a Laming Fellow at Queen's College, Oxford 1932-33 and then entered journalism on the editorial staff of The Times. From 1936-8 he was an Assistant-Master in Winchester College, after which he edited The Times Educational Supplement for two years. At the beginning of World War II he was serving in the Army's Intelligence Corps, but Admiral John Godfrey, the Director of Naval Intelligence, on learning that his duties were not very demanding, extracted him from that position and obtained a commission for him as a Lieutenant (Special Branch) RNVR. After a short time in the German Section (NID1) he became the First Secretary of the Future Operations (Enemy) Section (FOES), a small body set up initially under Captain T.H. Troubridge to study and forecast future strategy. Towards the end of 1940 he was transferred to NID's Co-ordinating Section (NID 17) which worked in the famous Room 39 next to the office of the DNI. In 1941 he became head of the Naval Propaganda sub-section NID 17Z, which, with the Political Warfare Executive (PWE) dealt with 'black' and 'grey' wireless and other propaganda directed against the German Navy. He continued in that capacity until the end of the war.

After the return of peace he became Assistant Editor (Foreign) for The Economist 1947-54, then Deputy Editor of The Daily Telegraph 1954-60. A year as a Visiting Fellow at Nuffield College, Oxford, followed, after which he returned to Fleet Street as Editor of the newly born Sunday Telegraph 1961-66. On resigning that very arduous post he decided to become a free- lance writer chiefly on the subject with which he was most familiar -namely Naval Intelligence. His first book Room 39 (Weidenfeld & Nicolson, 1968) was very successful, and despite the fact that he was handicapped by the absolute ban on publication of anything to do with cryptography then in force, still provides an excellent all-round view of the Naval Intelligence organisation and work in World War II. He had several other books in hand or projected when he was killed in a car accident on 10th January 1971. He had promised to deposit more papers in Churchill College, but the promise was frustrated by the Ministry of Defence removing the remainder after his death.