Born 26 October 1927. St.John attended Monmouth from 1939-45 and was in Town House and then Country House. He was in the Air Training Corp Cadets, however, finally joined the Royal Navy.

His career at sea in the Royal Navy was completed as Commander of HMS Plymouth in 1970. He was then on shore duty with NATO until his retirement from the Royal Navy in 1979. From there the family emigrated to Australia and after a stint with the Royal Australian Navy in Canberra, he retired to a property in central Queensland where he ran cattle. It was here in December 2020 that St.John passed away peacefully at home.