Hamilton, Gustavus (d. c.1691), soldier and governor of Enniskillen, was born probably in Sweden, elder of two sons of Ludovick Hamilton, a mercenary colonel in the Swedish army, and his Swedish wife. His grandfather was Archibald Hamilton (qv), a Scottish-born archbishop of Cashel. His career has been frequently confused with that of his contemporary namesake, Gustavus Hamilton (qv) (1642–1723), later 1st Viscount Boyne, who was governor of Coleraine, Co. Londonderry, in 1689. After he came to Ireland, in 1678 he was commissioned quartermaster in a troop of cavalry, apparently as part of an arrangement whereby command of the troop passed from his uncle, Hugh Hamilton (qv), 1st Baron Glenawley, to Lord Collooney. He was cornet in the same troop in the earl of Ossory's regiment at the accession of James II (qv), but resigned his commission in 1687 and retired to his estate at Monea in Co. Fermanagh, where he was a JP. In December 1688 he led a mounted force of a hundred men to support the protestants of Enniskillen when they resisted the entry of a Jacobite garrison. As a result of this, and presumably also on the strength of his military background, he was elected governor of Enniskillen and took up residence in the castle with his family. He played an active part in organising the successful Williamite defence of Enniskillen and the Erne valley, although as a soldier he was soon overshadowed by his capable and resourceful second-in-command, Thomas Lloyd of Co. Roscommon, a former dragoon officer.
Initially Hamilton raised two companies of foot in the town, and a further company with a troop of horse from his own estate and the adjoining countryside. An influx of protestant refugees from north Connacht and elsewhere enabled further companies to be added, and in January 1689 the infantry was formed into a regiment, of which Hamilton was elected colonel with Lloyd as lieutenant-colonel. By July 1689 the Enniskilleners mustered thirty companies of foot, seventeen troops of horse, and a few troops of dragoons. Available firearms were collected, repaired, and distributed, and pikes manufactured, many from scythes. Improvements were made to the defences of Enniskillen, including the addition of an earthen fort erected at Hamilton's personal expense. Communications were established with other centres of resistance in Ulster, but Hamilton refused to abandon the Erne valley when invited by Robert Lundy (qv) to join him at Derry. William (qv) and Mary were proclaimed in Enniskillen in March and delegates sent to England to solicit military assistance and commissions. The same month, the Enniskilleners beat off a Jacobite attack on Crom Castle, their principal stronghold at the east end of the Erne Valley, after Hamilton sent reinforcements by water. In April, with Hamilton's approval, Lloyd commenced a series of enterprising and successful raids against the Jacobite quarters. By contrast Hamilton's only foray, a somewhat foolhardy expedition in early June to relieve Derry, was a failure. The operation was abandoned after his force of 2,000 men reached Omagh, where it was held up by a Jacobite garrison, ran short of supplies, and was in danger of being taken in the rear by a Jacobite force which was marching towards Derry.
Provoked by the resistance of the Enniskilleners, the Jacobites now mounted a series of attacks against them. The first, under Sutherland, was beaten off by Lloyd, but a second in July, under Berwick (qv), threatened Enniskillen from the north and came close to success. Two companies of Enniskilleners were routed, and Hamilton was later accused of failing to come to their support. Luckily, Berwick was recalled to the operations at Derry before he could press home the attack. Hamilton during this operation burned several houses in the neighbourhood of Enniskillen, including Castlecoole, in order to deny them to the Jacobites. Meanwhile the Enniskilleners had established contact with Percy Kirke (qv), the commander of the Williamite expedition sent from England to relieve Derry. The outcome was that commissions were issued for the Enniskillen forces to be formed into six regiments, and William Wolseley (qv), an English professional soldier, was sent to command them, accompanied by some other officers, a small train of artillery, and a supply of arms and ammunition. Hamilton had the consolation of being appointed colonel of the senior infantry regiment and also retained the governorship of Enniskillen, where he largely remained for the rest of his career. The excuse of illness, possibly masking resentment over his loss of command, led to his absence from the Enniskilleners’ comprehensive victory over Mountcashel (qv) at Newtownbutler on 31 July.
In the autumn Hamilton travelled to England to be fêted with Governor George Walker (qv) of Derry, and used the occasion to successfully lobby King William and parliament for the Ulster regiments to be taken fully on to the establishment and given the same pay as the rest of the army, an arrangement hitherto resisted by the duke of Schomberg (qv), the new commander of the Williamite forces in Ireland. Having returned to Enniskillen, he was obliged to account in person to Schomberg for Mountcashel's escape from custody there at the end of 1689. He sought assistance from Schomberg when his garrison ran short of food and money before the campaign of 1690. He participated in Wolseley's attack on the Jacobite quarters in Cavan in March 1690, but seems to have been absent from the battle of the Boyne. He was dead, apparently from natural causes, by July 1691. In 1697 the treasury lords recommended his widow to King William as an object of compassion.