John Gwilym was born in 1918 and was brought up in Holyhead, Anglesey, within the characteristic culture of North Wales, which in his boyhood was still relatively unaffected by modernisation. His father introduced him to country sports, but his mother diverted him towards more intellectual interests; however, she died when Gwilym was 15, which affected him deeply. At his grammar school, where he was captain of cricket, he received a good grounding in science subjects but was uncertain about his future direction.