In 1696, Kerr was commissioned Captain in 'McGill's Regiment of Foot;' this was disbanded in 1697 following the Treaty of Ryswick and he transferred to Lord Jedburgh's Dragoons, a regiment owned by his eldest brother. When the War of the Spanish Succession began in 1702, he joined George Macartney's Regiment of Foot, which served in Flanders; in January 1706, he became Colonel of a newly recruited unit, Lord Mark Kerr's Regiment of Foot. Sent to Spain to support the Hapsburg candidate, Archduke Charles, it fought in the April, 1707 Battle of Almansa. The Anglo-Portuguese infantry were left isolated when their cavalry was routed and Kerr was wounded, while his regiment suffered severe losses. With the bulk of the surviving infantry, they retreated in good order to a defensive position 8 miles/12 kilometres from the battlefield. Out of ammunition, food and water, they surrendered the next day and the opposing commander, the Duke of Berwick, illegitimate son of James II of England, invited the captured officers to dinner. Their losses at Almansa meant the regiment was reformed in 1709 and disbanded two years later, as part of the reductions passed by the Tory government installed after the 1710 British General Election. Kerr was promoted Brigadier-General, but his juniors officers placed on half-pay; several of these, including Philip Lockhart and John Nairne, later joined the 1715 Jacobite Rising. Following the death of Thomas Farrington in October 1712, Kerr became Colonel of his regiment, Farrington's, later 29th Foot; this was sent to Gibraltar, where it remained for the next 30 years. Kerr may not have joined his regiment there, as in 1716 he was appointed Governor of Carrickfergus, and military commander for Counties Down and Antrim. During the War of the Quadruple Alliance, the Spanish supported the 1719 Jacobite Rising, which was quickly crushed. In retaliation, a British naval expedition under Lord Cobham landed 6,000 troops in the Spanish port of Vigo in October; they held it for ten days, destroyed vast quantities of stores and equipment, then re-embarked unopposed. Kerr commanded part of the landing force, which appears to have been his last active service command. In this period, the position of 'Colonel' implied ownership, not necessarily command and many delegated their duties to a subordinate. Kerr became Colonel of Stanhope's, later the 13th Regiment of Foot in 1725, made Major-General in 1727 and Governor of Sheerness in 1729. In 1732, he transferred his Colonelcy to Philip Honywood's Dragoons, later 11th Hussars, promoted Lieutenant-General in 1735, then full General on 26 February 1743. When the 1745 Jacobite Rising began in August 1745, he was Governor, Berwick-upon-Tweed. He reportedly joked Sir John Cope, defeated at the Battle of Prestonpansin September, was the first general to bring news of his own defeat but this seems to have been a later invention by the author Sir Walter Scott. Kerr was appointed Governor of Edinburgh Castle when government forces under General Handasyde retook the city on 14 November. He died in London in 1752.
