P/O St.John joined the RAF on a Short Service Commission in September 1937, being appointed Acting Pilot Officer on 28th November 1937.
He was posted to 3 FTS South Cerney on 11th December and then joined 87 Squadron at Debden on 9th July 1938, becoming a Pilot Officer on 27th September.
Peter arrived at 5 OTU Aston Down on 23rd March 1940 and after converting to Spitfires he was posted to his only squadron; the Tigers, at Hornchurch on 20th April. joined No 74 Squadron on 7th May, 1940, from No 501 Squadron.
Peter was killed on 22nd October, 1940. His Spitfire II (P7431) was shot down by a Bf 109. His aircraft crashed at South Nuttfield, Surrey at 15:30hrs.