Although RM Officers were used as "Observers" in the Great War they were not trained in radio and navigation since they were literally passengers who were observing. They did not have the navigational responsibility required when working from aircraft carriers over the sea. So far as is known only two RM Officers have qualified as Observers since the Great War, namely Captain K. L. Ford and Johnston and both were killed.
On completion of training Johnston was posted to 812 Squadron. This squadron was the pioneer F.A.A squadron working under R.A.F Coastal Command. From May 1940 until March 1941 they operated with Swordfish fitted with auxiliary fuel tanks in the rear cockpit and carrying a crew of only two. Their targets were usually oil tanks at Calais and invasion barges at Rotterdam.
On 4 August 1940, only ten days after joining the squadron he was reported missing, presumed dead. The official notification was "Killed in action off the English Coast".