Mary Trevor was buried in St John the Baptist's chapel in Westminster Abbey on 29th March 1780 aged 71. She was unmarried. Until the 19th century there was nothing to mark her grave but her name, and those of others in the vaults below the chapel who had no inscriptions, was added to a stone just inside the doorway. This is on the same stone as the Earl of Essex but is now rather faint. Her section reads:
In graves eastward..Mary da. of John Morley Trevor Esq. 1780
She was the third daughter of John Morley Trevor of Plas Teg, Trevalyn in Flintshire in Wales and of Glynde in Sussex. Her mother was Lucy daughter of Edward Montagu of Boughton, Northamptonshire. She was baptised at Glynde on 1st November 1708, her father dying several years later on 12th April 1719. Her only surviving brother was John Trevor who died in 1745. Her sister Ann married the Hon. George Boscawen, a son of Hugh, 1st Viscount Falmouth, in the Abbey on 3rd February 1743. She died in Bath in 1783.
Wills of her father, herself and her sister are at The National Archives, Kew, Surrey
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