Of Cornish descent, John Hewitt was born in London. His father's position as an engineer in the prison service meant that the family was constantly moving house, and John attended several different schools. As a teenager he developed a passion for aircraft and ships. In 1939, at the age of 16, he began an apprenticeship in the RAF. His work as an airframe fitter during the Second World War took him to South Africa, Rhodesia and India. After he was demobbed in 1953 he and his wife Margaret settled in Cornwall, near Newquay.

As a consequence of contracting tuberculosis during the war, John was hospitalised and had one lung removed. This illness proved to be a turning point. While convalescing, he came across some oil paints and began to depict his beloved aircraft. In the early 1960s this self-taught artist exhibited his works at the Society of Aviation Artists in London. He then turned his attention to Cornish subjects, primarily seascapes, which successfully captured the movement of the waves and the light catching the water. He started to earn his living from his art, opening his own gallery at the Pavilion Buildings in Rock in 1968, supplementing the summer sales with commissioned work out of season. He recovered his health and continued to work well into his late 70s, finally closing his gallery around 1996.