James Wemyss was born in 1748 to James Wemyss and Elizabeth Wylie. He was baptized on 19 July 1749, in Fife, Scotland. He was commissioned as an ensign in the 40th Regiment of Foot in 1766. He purchased a lieutenancy in 1767, and a captaincy in 1771, commanding a company of grenadiers. The 40th Regiment of Foot arrived in Boston, Massachusetts in 1775, during the Siege of Boston. Wemyss commanded a battalion of grenadiers, and prepared to attack Dorchester Heights but never did.[4]
In 1776. he served as an aide-de-camp to General James Robertson and was present at the Battle of Long Island. In 1777, he served under William Tryon during the Battle of Ridgefield and was slightly wounded.[5] He later took command of the Queen's Rangers and took the provincial rank of major. He led the corps at the Battle of Brandywine, where we was wounded once again. Shortly after this he resigned command, because he was not promoted, and was replaced by John Simcoe. In August 1778, Wemyss became a major in the 63rd Regiment of Foot, and was around the area of Stony Point, New York.
Wemyss was sent to South Carolina, being present during the Siege of Charleston in 1780. After the cities fall, he was sent to Georgetown, South Carolina where his job was to reinstall British authority. During 1780, he conducted operation along the Pee Dee River, where he carried out expeditions against Francis Marion. In November 1780, he attempted to ambush an American force led my Thomas Sumter at the Battle of Fishdam Ford. He was too aggressive in his approach and attacked at 1 o'clock in the morning, not waiting till morning to attack. Not being able to see well, his men walked right into the raised defenses of the patriot force, who had been told by coincidence to be up and at guard that night. Wemyss was wounded and captured during this battle, with his wounds were severe enough to end his field service during the war.[6]
After recovering he held administrative roles, carrying dispatches and was appointed Deputy Adjutant General in the southern theater. He also negotiated prisoner exchanges in South Carolina during the British evacuation in 1783.[7]
In 1780 during his campaign with Charles Cornwallis, Wemyss reported himself that he had burned 50 houses and plantations. He also executed deserters and militia defectors, and regularly destroyed property suspected of use by patriot rebels. In one instance, a patriot rebel named Adam Cusack was executed under Wemyss's orders. This earned him a reputation as a ruthless and rigid officer, 2nd only to Banastre Tarleton. Wemyss is also known to have burned a church in Indiantown, South Carolina, being noted as especially harsh by Banastre Tarleton.[8] He is also associated with Christian Huck.[9]