On 18 May 1878, at St Augustine's Church, London, he married Candida Louisa, a daughter of Signor Vincenzo Bartolucci of Cantiano, Italy, and they had three sons and one daughter.
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He went on to become Lord High Commissioner to the General Assembly of the Church of Scotland and was appointed a Knight of the Thistle.
His paternal grandfather was James Hay, 15th Earl of Erroll, son of William Boyd, 4th Earl of Kilmarnock (who was attainted with his titles forfeited in 1746).
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William George Hay, 18th Earl of Erroll KT, GCH, PC (21 February 1801 – 19 April 1846), styled Lord Hay between 1815 and 1819, was a Scottish peer and politician.
William Shakespeare | William Laud | William Blake | William | William III of England | William Morris | William McKinley | William Howard Taft | William Ewart Gladstone | William the Conqueror | William S. Burroughs | William Shatner | William Faulkner | William Randolph Hearst | William Wordsworth | William Tecumseh Sherman | William Hogarth | Prince William, Duke of Cambridge | William Penn | William Jennings Bryan | William Gibson | William Wilberforce | William James | William Makepeace Thackeray | Fort William | William Hanna | William Hague | William III | William Hurt | William Walton |
He was devoted to his wife Lady Agnes Duff, the youngest daughter of James Duff, 5th Earl Fife by his wife, Lady Agnes Hay, daughter of William Hay, 18th Earl of Erroll and granddaughter of King William IV by his mistress, the actress Mrs. Dorothy Jordan.
The only nominee was William Hay, DUP member for Foyle and he was elected unopposed.
Their eldest daughter Mary married Sir William Hay of Yester, another daughter Geillis married Sir Robert Logan of Restalrig, and Margaret married Roger Kirkpatrick of Closeburn in Niddsdale.