 Educated at Bishop's Stortford College, Bonsall went on to study modern languages at St Catharine's College, Cambridge before joining the Government Code and Cypher School at Bletchley Park.From 1940, Bonsall served in the German Air Section under Josh Cooper, studying the Luftwaffe.In 1942 he helped to create a series of daily reports known as the BMP (from the initials of its three co-creators, Bonsall, Moyes and Prior).