He was appointed temporary Deputy Military Secretary, India from 21 December 1939 to 17 August 1940. From 18 August 1940 to 13 January 1942 he commanded the 8th Indian Infantry Brigade in Malaya. Key's brigade were the first British troops to face the Japanese when they invaded Malaya in December 1941. When Major General David Murray-Lyon was dismissed from command of the 11th Indian Division Key replaced Brigadier Archibald Paris as commander of the division. He proved an able commander but the situation in Malaya was at this point beyond salvaging and the British Army surrendered at Singapore on 15 February 1942 to a numerically smaller Japanese force. Key spent the remainder of the war as a prisoner of war in Japan.