Johns' graduation coincided with the start of World War II. For the duration of the war, he taught physics, mathematics, radar systems, and radio navigation to newly recruited airplane pilots as part of the British Commonwealth Air Training Plan. Based on his radiography and physics experience, Johns was also involved in non-invasive X-ray testing of metal aircraft castings. A meeting in August 1946 with William Valentine Mayneord, while Mayneord was at the Atomic Energy Project at Chalk River, Ontario, contributed to Johns's making a career in medical physics.