 Promoted to rear admiral on 19 April 1911, he became Commander of the 2nd Battle Squadron of the Home Fleet in October 1912.Wemyss served in the First World War, initially as commander of the 12th Cruiser Squadron in the Channel Fleet and then, after being despatched to Lemnos with a brief to prepare the harbour of Moudros for operations against the Dardanelles in February 1915, as Governor of Moudros.He commanded a squadron for the British landings at Cape Helles on 25 April 1915 and also gave support for the British landings at Suvla Bay on the Gallipoli Front on 9 August 1915.