The future Admiral of the Fleet was born on 12 April 1864. His father was James Hay Erskine-Wemyss of Wemyss Castle, Fife, in Scotland, and his mother Millicent Ann Mary, daughter of Lady Augusta Erskine who was a daughter of the Duke of Clarence (later King William IV) by Mrs Jordan. The naval strain in his ancestry was strong on the maternal as well as the paternal side. He joined the training ship Britannia on 15 July 1877 at the same time as his Royal cousins the future Duke of Clarence and the future King George V. Wemyss's place on entry was 13th out of 42 cadets, and as he passed out of the Britannia 18th two years later he does not appear to have shown early promise. In 1879 he went to sea as a Midshipman, and he and the Royal Princes served in the corvette Bacchante during a three year cruise round the world. After a brief spell in the Northumberland (Channel Squadron) Wemyss joined the corvette Canada on the North America and West Indies Station, with Prince George again as a shipmate. He was promoted Sub-Lieutenant in August 1884 and returned home to undergo courses for Lieutenant's rank, to which he was promoted in March 1887. His next appointment was to the Royal Yacht Osborne for two years, after which he served as Flag Lieutenant to Admiral Tracey, second in command of the Channel Squadron.

From March 1890 to June 1893 Wemyss was on the cruiser Undaunted under Captain Lord Charles Beresford, on the Mediterranean Station, after which he returned to the Channel Squadron for two years in the flagship of Admiral Edward Seymour. A period as First Lieutenant on the Astraea (2nd class cruiser) in the Mediterranean followed, after which he was appointed First Lieutenant of the Royal Yacht Victoria and Albert in 1896. Two years later he was promoted Commander at the fairly early age of 34.

The outbreak of the Second Boer War produced service as Commander of the cruiser Niobe 1899-1900 in the South Africa Station, after which he became second in command of the specially commissioned liner Ophir which was to convey Prince George (now Duke of York) to the overseas Dominions. Wemyss was specially promoted Captain in 1901 after the end of that service.

The new Selborne Scheme for the entry and training of naval cadets was resumed in December 1902, and Admiral Sir John Fisher, then 2nd Sea Lord, had selected Wemyss as one of the principal agents to bring it into force. From August 1903 to September 1905 he was Captain of the newly established Royal Naval College Osborne on the Isle of Wight. He was then appointed to the cruiser Suffolk on the Mediterranean Station, when Lord Charles Beresford was Commander-in-Chief, from 1905 to 1908 and in 1909 became Commodore of the RN Barracks, Devonport.

In 1910 he resumed his association with the Royal family by commanding the specially commissioned liner Balmoral Castle which was to take the Duke and Duchess of Connaught to South Africa for the opening of the first Parliament of the Union. It had originally been intended that the Prince of Wales [earlier the Duke of York, later King George V] should undertake this duty, but the death of Edward VII necessitated the change. In April 1911 Wemyss was promoted Rear-Admiral, and in the following year he was given command of the 2nd Battle Squadron in the Atlantic Fleet.

On the outbreak of war in August 1914 Wemyss hoisted his flag in the cruiser Charybdis in command of the 12th Cruiser Squadron, which was charged with ensuring the safe passage of the British Expeditionary Force to France. After that important task had been successfully accomplished his Squadron was ordered to Canada to escort the first contingent of 30,000 soldiers from that country to England. In 1915 Wemyss was selected as Governor of the Aegean island of Lemnos with responsibility for creating a new naval base at Mudros for the impending assault on the Dardanelles. In April he began to take an active part in the Gallipoli operations with his flag in the cruiser Euryalus. His services during the hazardous and costly landings were invaluable, and Wemyss showed particular tact in all his dealings with the military authorities.

In November 1915, when the naval Commander-in-Chief, Admiral Sir John de Robeck was on leave, Wemyss was appointed Acting Vice-Admiral and shifted his flag to the battleship Lord Nelson. In that capacity he, like Commodore Roger Keyes, de Robeck's Chief of Staff, pressed for renewal of the naval attack; but military opinion decided otherwise, and Wemyss was forced, most unwillingly, to plan and execute the evacuation of the forces landed at Suvla and Anzac, soon followed by evacuation of the forces on Cape Helles.

In January 1916 Wemyss was appointed KCB for his services in the Dardanelles campaign, and became Commander-in-Chief, East Indies and Egypt Station with his flag still on the Euryalus. He co-operated very successfully with the Army in the defence of Egypt and its subsequent advance into Sinai; but his attempt to use his squadron to relieve the besieged Major-General Sir Charles Townshend in Kut-el-Amara [later Al-Kut, Iraq] was unsuccessful. Wemyss's service at this time brought him into close contact with the members of the 'Arab Bureau' in Cairo (General Sir [Francis] Reginald Wingate, Ronald Storrs, Brigadier-General Gilbert Clayton, etc) and he co-operated closely with them and with T E Lawrence [later T E Shaw, 'Lawrence of Arabia'] in fostering the Arab revolt against the Turks. He also gave full support to General Sir Edmund Allenby's advance into Palestine. He evidently won the confidence of, and gained golden opinions from, all the multifarious authorities then involved in the intricacies of the war in the Middle East.

In June 1917 Wemyss was promoted Vice-Admiral and it was first intended that he should take command of all British ships in the Mediterranean with his headquarters in Malta; but on returning briefly to London Sir Eric Geddes, the recently appointed First Lord of the Admiralty, preferred that he should become Deputy 1st Sea Lord - a newly created office. However, he soon found that Admiral Sir John Jellicoe, the 1st Sea Lord, was not prepared to delegate any substantial authority to him and a difficult situation was only resolved by Geddes dismissing Jellicoe at the end of the year and appointing Wemyss in his place. The change was completely successful, for Wemyss was fully prepared not only to organise a proper Naval Staff but to adopt much needed measures of decentralisation in the Admiralty. In all his dealings he showed a remarkable degree of statesmanship at this time. In June 1918 he was appointed GCB and in the following year he was promoted Admiral. He played a leading part in the negotiation of the Armistice of November 1918 and also the Treaty of Versailles, representing his country on the Allied Naval Council.

Trouble however arose over the desire of Admiral 1st Lord Beatty to succeed to the office of 1st Sea Lord, and over the titles and money awards made to the principal leaders of the British armed forces; and after an attempt to resign in August 1919 had been refused by Walter Long, the First Lord of the Admiralty, on 1 November he decided definitely to hand over to Beatty and left office. He was promoted Admiral of the Fleet and created Baron Wester Wemyss but he received no money award and, despite his undoubted success in several difficult posts his services were never again utilised in any capacity by the post-war governments.

In 1903 Wemyss married Victoria, daughter of the distinguished diplomat Sir Robert Morier, by whom he had one daughter. He died on 24 May 1933.