Law was elected as Member of Parliament (MP) for Kingston upon Hull South West in the general election of 1931 and held the seat until 1945. In 1940 he was appointed Financial Secretary to the War Office. He was then transferred to the post of Parliamentary Under-Secretary of State for Foreign Affairs until 1943. While in the latter post he took part in the Bermuda Conference on the fate of European Jewry and was sworn of the Privy Council in the 1943 New Year Honours.