After a short period as first lieutenant in the old destroyer , from September 1939 to April 1940, Gretton was appointed at very short notice as first lieutenant in the large modern destroyer and saw action at the Second Battle of Narvik during the Norwegian Campaign. He was given command of the old destroyer in 1941 and served in the North Atlantic. Promoted to lieutenant-commander on 1 June 1942, he was given command of the marginally newer destroyer and returned to the Mediterranean. He took part in Operation Pedestal, the Malta convoy operation in August 1942, and sank the Italian submarine "Dagabur" by ramming. Promoted to commander on 31 December 1942, he was given command of the destroyer , as Senior Officer Escort to Escort Group B7, based in Derry.