In 1938, he was elected to the Northern Ireland House of Commons as a Unionist member for Belfast, St Anne's, which he represented until his retirement from Parliament in 1969.
[2] He served as Parliamentary Secretary to the Ministry of Home Affairs from 1938 to 1940, when he resigned[3] in protest at the failure to extend conscription to Northern Ireland during the Second World War.
[2] While Parliamentary Secretary to the Minister of Home Affairs (1938–40), Warnock, despite being advised by defence experts in Great Britain to prepare for German aerial attacks, decided to cancel orders previously placed for fire-fighting equipment and to recommend not building air raid shelters to protect either the civilian population or workers in factories, even those in the vital Harbour Estate area containing the shipyards and aircraft factories.
Warnock believed that Belfast was too remote for German bombers to reach, and, in any case, they would pass more attractive targets en route.
(see Brian Barton 'The Belfast Blitz: The City in the War Years', Ulster Historical Foundation, 2015. pages 38–41).