During the First World War he served as commander of the 12th Cruiser Squadron and then as Governor of Moudros before leading the British landings at Cape Helles and at Suvla Bay during the Gallipoli campaign.

[2] Promoted to lieutenant on 31 March 1887,[3] he joined the Royal Yacht HMY Osborne in October 1887 and then transferred to the battleship HMS Anson in the Channel Squadron in September 1889.

[2] He was posted to the President for special service on 1 January 1901,[5] but was shortly thereafter invited by the Duke of York to become second-in-command of the ocean liner SS Ophir during the royal cruise March–October 1901 to open the Parliament of Australia and visit Commonwealth countries to thank them for their support during the Boer War.

[8] He was briefly posted to the President for temporary service at the Admiralty in October 1902,[9] but the following month was on 25 November 1902 appointed in command of the old battleship HMS Superb, in Fleet Reserve at Portsmouth.

[18] Promoted to vice-admiral on 6 December 1916,[19] he was appointed a Commander of the French Legion of Honour on 23 March 1917[20] and a Grand Officer of the Order of the Crown of Italy on 11 August 1917.

[22] 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.

[25] He was also promoted to full admiral on 21 February 1919,[26] awarded the Grand Cross of the Romanian Order of the Crown on 17 March 1919[27] and presented with the American Distinguished Service Medal on 16 September 1919.

[30] After attending the Paris Peace Conference as Britain's naval representative and arranging for the end of the food blockade and hence unblocking the negotiations,[31] Wemyss resigned in November 1919 following persistent calls for Sir David Beatty to be given his job.