On 6 June 1673 when he was about twelve years old, his father died from a wound received at a sea-fight with the Dutch in the Third Anglo-Dutch War. He was compensated by an appointment as an extra groom of the bedchamber on 18 April 1680. His father had predeceased his grandfather who still held the land of Donalong between Strabane and Derry in Ireland. At his grandfather's death in 1679, he inherited the land and succeeded to his grandfather's baronetcy, i.e. Baronet Hamilton of Donalong, but did not use the title.
Marriage and children
He married Elizabeth Reading, daughter of Sir Robert Reading, 1st Baronet and Jane Hannay, widow of Charles Coote, 1st Earl of Mountrath, in January 1684. Charles II issued a warrant on 22 January 1683/4 to create Hamilton "Baron Hamilton of Bellamont", county Dublin, in the Irish peerage, but it never passed the seals. The couple had 14 children of which 10 survived into adulthood:
Rev. Hon. Francis Hamilton, married and had issue;
William, was lost aboard HMS Royal Anne Galley; and
Charles, MP, married and had issue.
Expedition to Derry
His post in the bedchamber ended with the King's death in 1685. He had entered a career in the army and held a commission in the English army of the new king, James II. In 1688 at the Glorious Revolution he sided with William. In spring 1689 when war menaced in northern Ireland, he was sent to Derry with provisions in order to prepare the city for a likely siege. On 21 March 1689 he arrived at Derry from England with two ships: the frigate and the merchantman Deliverance, bringing gunpowder, munition, weapons, and £595 in cash. These provisions were to be crucial during the Siege of Derry. He also brought the commission from King William and Queen Mary that confirmed Colonel Robert Lundy as Williamite governor of the town. He therefore helped to defend Derry. His uncle Richard Hamilton, lieutenant-general in the Irish Royal Army, attacked it.
In June 1701, upon the death of his second cousin Charles, he succeeded to the titles of Earl of Abercorn and Baron Hamilton of Strabane and entered into the possession of the corresponding lands. About half a year later, on 2 December 1701, William rewarded Abercorn with the titles of Viscount Strabane and Baron Mountcastle, in the Peerage of Ireland. The first was an enhancement of his title of Baron of Strabane. He took his seat in the Irish House of Lords on 21 September 1703, and in the Parliament of Scotland on 3 October 1706. By April 1711, he had been appointed also to the Privy Council of Ireland.
Death, succession, and timeline
Abercorn died on 28 November 1734 at the age of 73 and was buried on 3 December in the Ormond vault of the Henry VII Chapel in Westminster Abbey. The Ormond Vault was opened in 1868 and was found to be filled with many coffins stacked one over the other. Their number was estimated at 59. Individual identification beyond the top layer was not attempted. Abercorn's remains may well be there. He was succeeded by his eldest son James as the 7th Earl. His wife died on 19 March 1754.