John Masterman was born in 1891. After studying history at Worcester College, Oxford, Masterman went to Heidelberg to improve his German. Arrested in 1914 he spent the whole of the First World War interned as an enemy alien.

After the war Masterman taught history at Oxford University. By this time he had become a part-time member of MI5 and was active in the campaign against the trade union movement during the General Strike. He also used his position to recruit secret students as agents. This included Dick White, the future head of MI5.

During the Second World War Masterman became chairman of the XX (Double-Cross) Committee. Arthur Owen, an agent working for Abwehr, was arrested and he eventually agreed to become a double-agent. As well as sending false information to Nazi Germany, Owen also kept the committee informed about the arrival of German agents in Britain. Between September and November 1940 a total of 21 German agents were arrested by Special Branch officers as a resut of this scheme.

In 1972 Masterman published an account of the XX (Double-Cross) Committee in his book The Double-Cross System in the War of 1939-45. The book included details of 120 double-cross agents working in the war.

John Masterman died in 1981.