Harrison joined the army and was a cornet in the 4th Dragoon Guards in 1697, ADC to the Duke of Ormonde, Lord lieutenant of Ireland before 1705, captain and lieutenant colonel in the 1st Foot Guards in 1705, brevet colonel in 1707 and colonel of the 6th Foot from 1708 to 1716. He was adjutant-general in Spain in 1708 and brought back Lord Stanhope’s despatches after the victory at the Battle of Saragossa in 1710, for which he received £1,000 from Queen Anne. In 1715 he was adjutant-general in Scotland where he was present at the Battle of Sheriffmuir and brought the Duke of Argyll's despatches to George I, who gave him £500. He sold his regiment in March 1716.