In 1913, while living in Swaffham, and working as a sanitary inspector, Johns enlisted in the Territorial Army as a trooper in the King's Own Royal Regiment (Norfolk Yeomanry). The regiment was mobilised in August 1914 and was sent overseas in September 1915, embarking on RMS "Olympic". The Norfolk Yeomanry fought (as infantry) at Gallipoli until December when they were withdrawn to Egypt. In September 1916 Johns transferred to the Machine Gun Corps. While serving on the Macedonian front in Greece he was hospitalised with malaria. After recovering, he was commissioned into the Royal Flying Corps (RFC) in September 1917 as a temporary second lieutenant and posted back to England for flight training.