Montgomery, William (1633–1707), historian, was born in October 1633, probably at Aghintain, Co. Tyrone, only child of Sir James Montgomery of Rosemount, Co. Down (a younger son of Hugh, 1st Viscount Montgomery), and Catherine, daughter of Sir William Stewart of Newtownstewart, Co. Tyrone. He witnessed the outbreak of the 1641 rebellion while staying at the Stewart house in Tyrone and spent the next few years at various places of safety in Scotland and Ireland. In 1649 he entered Glasgow University, but after Cromwell's victory over the Scots at Dunbar (1650) he moved to the University of Leyden. After the death of his father (June 1652), he went to London to petition for the return of his father's estate, which had been confiscated by the Cromwellian regime. In 1653 he returned to Co. Down, where he became involved in a series of legal disputes relating to the recovery of the estate. After the restoration of the monarchy (1660) he regained the family property, including Rosemount House. He was elected MP for the borough of Newtownards in 1661, after failing to win one of the Co. Down seats, and was custos rotulorum for the county 1663–5. As well as having his estates restored, he was awarded nearly £10,000 as compensation for his father's losses during the civil war. Although he was made a trustee for the '49 officers’, he recovered only a small portion of this sum. He was in London 1664–5, representing his cousin the 2nd earl of Mount-Alexander (qv). It would appear that he spent the rest of his life looking after his estate, and apart from a term as high sheriff of Co. Down (1670) he took no further part in public life. Like many other Irish protestants he fled to Scotland in 1689, returning the next year.
Apart from two Latin dissertations printed in Leyden in 1652, none of Montgomery's works was published during his lifetime. He was, however, a prolific writer and by the end of his life had compiled several manuscript volumes of poetry and religious reflections. In 1683 he wrote a topographical description of the Ards peninsula at the request of William Molyneux (qv), who was planning to publish an atlas of Ireland, but this project came to nothing. He did not begin his account of the Montgomery family in Ireland until 1697. A crippling attack of gout gave him the leisure to undertake this project, and he may also have been motivated by a fire at his house at Rosemount in 1695 which resulted in the loss of many family documents and relics. The ‘Memoirs of the Montgomerys of Ireland’ consist of accounts of the lives of Hugh Montgomery (qv), 1st Viscount Ards, and his descendants, including the author and his father Sir James Montgomery of Rosemount. There are also accounts of collateral branches of the family in Ireland and Scotland, and of his cousins the Savages of Portaferry. Sections of the memoirs, including the life of the 2nd Viscount Montgomery, have not survived or are incomplete. Despite these gaps, the memoirs give a comprehensive and colourful account of the Montgomery family in Ireland from the plantation to the end of the seventeenth century. The main sources are family tradition and William Montgomery's own experiences. He had access to some documentary material from the earlier period, but in the main he relied on the accounts he received from family members and retainers. Very little use is made of secondary sources. The memoirs give a contemporary, or near-contemporary, account of Ulster from the perspective of one of the most important of the new settler families. Montgomery makes no pretence of objectivity and he sometimes falls into anachronism; however, the memoirs remain one of the most important sources for the history of seventeenth-century Ulster.
Montgomery wrote for the edification of his family and friends rather than for publication. His manuscripts were made available to John Lodge (qv), who based his entry on the earls of Mount-Alexander in his Peerage of Ireland (1754) on them. Extracts from the manuscripts were published in the Belfast News Letter (1785–6, 1822) and in a volume edited by James MacKnight (qv) in 1830. In 1869 George Hill (qv) produced a new edition which included most of the surviving manuscripts along with extensive scholarly notes.
Montgomery married (1660) his cousin Elizabeth, daughter of Hugh, 2nd Viscount Montgomery, and Jean Alexander. They had one son, James. William Montgomery died 7 January 1707 and was buried at Greyabbey. Most of his surviving personal and business papers are in the possession of his family. A microfilm copy is available in PRONI, which also holds some of his surviving manuscripts among the papers of the Nugent (formerly Savage) family of Portaferry.