Appointed Commander-in-Chief Coastal Command with the acting rank of air marshal on 5 February 1943, Slessor had at his disposal sixty squadrons, two of which were equipped with B-24 Liberator heavy bombers. He was credited with doing much to turn the tide of the Battle of the Atlantic in the Allies' favour by employing his thinly stretched long-range bomber force against the U-boat threat, in close cooperation with naval forces. Promoted temporary air marshal on 1 June 1943, he was advanced to Knight Commander of the Order of the Bath in the 1943 Birthday Honours. Slessor became Commander-in-Chief RAF Mediterranean and Middle East in January 1944, and deputy to Lieutenant General Ira Eaker as Commander-in-Chief Mediterranean Allied Air Forces. In this role he conducted operations in the Italian Campaign and Yugoslavia, establishing the Balkan Air Force in the latter theatre. Slessor joined the Air Council as Air Member for Personnel on 5 April 1945. His rank of air marshal became substantive on 6 June. He was awarded the Grand Cross of the Greek Order of the Phoenix on 6 September 1946. His war service also earned him appointment as a Commander of the Belgian Order of Leopold on 27 August 1948, and a Knight Grand Cross of the Norwegian Order of St. Olav on 6 March 1953.