Having been invalided out of the British Army, he returned to Oxford and graduated in 1946 with a second-class honours Bachelor of Arts (BA) degree.
Unashamedly Francophile, Johnson devoted his academic career to improving relations between France and Britain.
He became an advisor to Margaret Thatcher on all matters concerning France, although his own political position was always something of a mystery.
A collection of essays in his honour was compiled by Martyn Cornick and Ceri Crossley under the title Problems in French History and published by Palgrave in 2000.
[3] Johnson was awarded a number of honours by the French government: National Order of Merit (1980); Commandeur of the Ordre des Palmes Académiques (1987); and Chevalier (1990) then Officier (1997) of the Legion of Honour.