Newnham was serving with the British Army Aid Group in British Hong Kong at the start of the Second World War. He was taken prisoner when the Japanese invaded Hong Kong in December 1941 and, with Captain Douglas Ford and Flight Lieutenant Hector Bertram Gray worked to contact British agents and organise a mass escape. The Japanese discovered the plan and arrested the trio, torturing them in Stanley Prison in an effort to gain more information. They refused to divulge any further names despite being beaten, starved and threatened with death. They were killed by firing squad in Sham Shui Prison Camp on 18 December 1943.