At the outbreak of World War II, he sent his wife and daughter to the United States and was commissioned into the British Army in 1940. Seconded to the Special Operations Executive, he was clandestinely sent to Crete and was captured in 1941, spending the rest of the war as a prisoner of war. In captivity, Hamson taught law to his fellow prisoners, and wrote an account of his captivity which was published by Trinity College after his death in 1989.