Following the outbreak of the Second World War, Sansom was sent to Washington, D.C. and then to Singapore, speaking to leading officials in the Royal Navy as an adviser on economic warfare. He was later appointed as a civilian representative on the Far East War Council. Evacuated to Java after the fall of Singapore, he was attached to the headquarters of General Archibald Wavell, but after the fall of Java to the Japanese Sansom was evacuated to Australia, and from there back to Washington, D.C., where he remained until the end of the war as a Minister Plenipotentiary attached to the British Embassy.