The sitter was the daughter of the Revd. Thomas Thistlethwaite, of Norman Court, Hampshire, and his wife Selina, daughter of Peter Bathurst, of Clarendon Park, Wiltshire. On 20th August 1777 she married Philip Stanhope, 5th Earl of Chesterfield (1755-1815) who had succeeded to the title in 1773. They lived at Bretby Hall, Derbyshire, and at Chesterfield House in London, the magnificent mansion built by the 4th Earl.
To commemorate their marriage in 1777, Chesterfield commissioned Gainsborough to paint full-length portraits of his wife and himself (figs. 1 & 2). From these sittings, Gainsborough executed a further two bust-length portraits of the Countess (the other and its companion of the Earl are in the Shirley Collection, Loch Fea, Ireland).
The present portrait, probably dating to 1778, is executed with remarkably free and fluent brushstrokes, most notably in the diaphanous wrap and blue satin dress, and exudes the confidence of an artist who had secured royal patronage with the exhibition of his portraits of the Duke and Duchess of Cumberland (London, The Royal Collection) at the Academy in the previous year.