James Wemyss was born in Scotland in 1748, probably at Edinburgh, and was baptized in Fife in 1749. He was the son of James Wemyss and Elizabeth Wylie Wemyss. Some accounts connect his family to the wider aristocratic Wemyss lineage, but the exact degree of relationship is not always clearly documented. The 1734 birth year and New Jersey birthplace currently attached to this database row belong to an erroneous identification and do not fit the British officer who served in the American Revolution.

Little is securely recorded about Wemyss’s childhood education. As a young man pursuing an officer’s career, he would have needed the social connections and financial means required to obtain a commission in the eighteenth-century British Army. At seventeen or eighteen he was commissioned an ensign in the 40th Regiment of Foot in April 1766. The purchase system made an officer’s advancement partly a matter of money and patronage, although competence and service still affected employment and reputation.

Wemyss purchased promotion to lieutenant in April 1767. His early military training took place within the regiment rather than at a formal staff college. He learned drill, company administration, discipline, accounts, correspondence, and the tactical handling of infantry. Promotion to captain followed in 1771, giving him responsibility for a company of grenadiers, troops selected for size, steadiness, and assault service. By his mid-twenties, he had spent most of his adult life within the professional army.

The 40th Regiment received orders for North America after conflict began in Massachusetts. Wemyss arrived at Boston in 1775, reportedly after the Battle of Bunker Hill, with roughly nine years of military experience. During the siege he prepared for possible operations against the American position on Dorchester Heights, though the anticipated assault did not occur. His arrival in the rebellious colonies closes his early career as a rising regimental officer. The staff service, Loyalist command, destructive southern raids, and harsh reputation for which he later became known all developed after this point.