The sitter was the daughter and co-heir of Richard Powis (or Powys) of Hintlesham Hall, Suffolk, by his wife, Lady Mary Brudenel, daughter of George, 3rd Earl of Cardigan.
She first sat to Reynolds with her sister Elizabeth in 1759, and it is possible that the present portrait dates to those sittings. There is also a portrait of her by Reynolds, datable to 1762 (private collection) in which she wears an elaborately ruched dress and a feather boa, in a feigned oval. She married James Viscount Stopford the same year, who later became the 2nd Earl of Courtown in 1770, and was created a peer in 1794 as Baron Saltersford of Saltersford, Chester.
Mary Lady Courtown was appointed Lady-in-Waiting to Queen Charlotte, and Fanny Burney (1752-1840) recorded in her famous diary that 'Lady Courtown has had a new place not merely given, but created for her. She was so useful and pleasant to the Queen at Cheltenham, that she has been appointed Lady in Waiting in the Country; by which means she will now regularly attend her Majesty in all country excursions, and during all her residences at Windsor and Kew. I am very glad of it, for she is constantly cheerful and obliging, and seems invariably in good humour and good spirits' (18 August, 1788).