 Following Sir Eric Geddes's decision to dismiss the First Sea Lord, Admiral Sir John Jellicoe, Wemyss was appointed Jellicoe's replacement in December 1917.As First Sea Lord he encouraged Admiral Roger Keyes, Commander of the Dover Patrol, to undertake more vigorous operations in the Channel, ultimately leading to the launch of the Zeebrugge Raid in April 1918.He was advanced to Knight Grand Cross of the Order of the Bath in the 1918 Birthday Honours and represented Britain at the Armistice in November 1918.