In August 1939, Johnson was finally accepted by the RAFVR and began training at weekends at the airfield Stapleford Tawney, a satellite airfield of RAF North Weald. There he received ground instruction on airmanship. Taught by retired service pilots of 21 Elementary & Reserve Flying Training School, Johnson trained on the de Havilland Tiger Moth biplane. Upon the outbreak of war in September 1939, with the rank of sergeant, Johnson entrained for Cambridge. He arrived at the 2nd Initial Training Wing to begin flight instruction. He was interviewed by senior officers in which he said his profession and knowledge of topography, surveying and mapping would make him more useful in a reconnaissance role. The wing commander agreed, but nonetheless, Johnson was selected for fighter pilot training and given the service number 754750 with the rank of Sergeant. Johnson and several hundred others were entrained for Cambridge and the 2 initial Training Wing. While assigned here Johnson learned basic military drill, sometimes given the slang name "square bashing".