He was born at Creevagh Beg, near Ballymahon, County Longford. His mother Elizabeth Morgan was the daughter of Robert Morgan of Cottlestown, County Sligo, and his wife Bridget Blayney of Castleblayney, County Monaghan. His father Anthony Marlay was a recent arrival in Ireland from Newcastle-upon-Tyne, where Thomas's grandfather Sir John Marlay had been a leading local politician, who served as both Mayor of Newcastle and its MP; he is chiefly remembered for his spirited defence of the town against the invading Scots army during the English Civil War. Beginning as small tradesmen, the Marlays had risen to become among the wealthiest coal exporters in Newcastle. George Marlay, Bishop of Dromore 1745-1763, was Thomas's younger brother, and gave his name to Marlay House, which was built by his son-in-law, David La Touche. David was an MP, a member of the Privy Council of Ireland, and first Governor of the Bank of Ireland. He married George's daughter, Elizabeth in 1761.
His most memorable trial as Lord Chief Justice was the so-called Annesley perjury case, Annesley v. Lord Anglesey, in 1745. This was one of several trials arising from the celebrated legal battle between James Annesley and his uncle Richard Annesley, 6th Earl of Anglesey as to which of them was the rightful holder of the Anglesey title and estates; the case is said to have inspired the novelKidnapped by Robert Louis Stevenson. The trial was notable for lasting twenty-two hours without a single break and Marlay, who would normally have had 2 or 3 colleagues to support him, conducted it singlehanded. The verdict went in favour of James, but he died before his uncle, without having regained possession of his estates. A workload as heavy as Marlay's was bound to affect his health, and from 1749 onwards he was too unwell to go on assize; he retired on health grounds in 1751. He died in Drogheda in 1756, while on a visit to his colleague Henry Singleton. He was a popular figure and his death seems to have been genuinely mourned: a Dublin paper published verses praising his gentleness, perfect manners and scholarship.
Family
Marlay rebuilt Celbridge Abbey, which is chiefly remembered as the home of Esther Vanhomrigh, the beloved Vanessa of Jonathan Swift. He married Anne de Laune, daughter of Charles de Laune, in 1707, and had ten children, of whom several died young. Their surviving children included Colonel Thomas Marlay; Richard Marlay, Bishop of Clonfert and later Bishop of Waterford; Anthony, who inherited Celbridge Abbey; and Mary, mother of the statesman Henry Grattan. , grandson of Thomas Marlay