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. When the Gallipoli Campaign was abandoned he was responsible for the successful re-embarkation of troops from Suvla Bay and from Ari Burnu on 20 December 1915. He was appointed a Knight Commander of the Order of the Bath in the 1916 New Year Honours.