In 1795 Hislop undertook a secret diplomatic mission to Germany at the request of the Prince of Wales and was subsequently promoted to lieutenant colonel in the 115th Regiment of Foot, returning to the 39th six months later. In 1796 his regiment was sent to the West Indies, and Hislop participated in the capture of the Dutch colonies of Demerara, Berbice, and Essequibo. After their capture, Hislop remained in the territory as military commander, raising a number of battalions of the West India Regiment. He moved to Trinidad as lieutenant governor in 1802 after the Peace of Amiens. In 1806 as Spain was then at war with Britain, Hyslop agreed with admiral Alexander Cochrane to provide some support to general Francisco de Miranda for a second attempt to invade Venezuela under Spanish rule. In the aftermath of the failed expedition, Miranda spent the next year in Port of Spain as host of Hyslop waiting for reinforcements that never came and return to London. In 1809, as British forces gathered for operations against the French Leeward Islands, Hislop joined them as a subordinate to Lieutenant-General George Beckwith and participated in the invasion of Martinique in February 1809 and the invasion of Guadeloupe in January 1810, commanding a division during the latter operation. He was promoted to major-general, and returned to Britain due to ill-health in 1811.