The sitter, Mary Elizabeth, was the only child of Richard Milles (1735-1820), of North Elmham, Norfolk, Member of Parliament for Canterbury (1761-1780), by his wife Elizabeth, daughter and heiress of the Rev. Thomas Tanner, whom he married on 9 October 1765. In November 1785, Mary Elizabeth married the Hon. Lewis Thomas Watson, eldest son and heir of Lewis Watson (1728-1795), formerly Monson of Rockingham Castle, who took the surname Watson in lieu of that of Monson in February 1745/46 and was created Baron Sondes of Lees Court, Kent on 22 May 1760. On his father being created Baron Sondes, Lewis Thomas Watson became the Hon. Lewis Thomas Watson and on the death of his father in March 1795 he became 2nd Baron Sondes of Lees Court. After the death of her husband in June 1806, the sitter married Major General Sir Henry Tucker Montresor. She is buried at Norton Church, Kent.
Waterhouse dated this painting to 1786 (op.cit), when Gainsborough was working in London. The sitter, who was 19 in that year, married the Hon. Lewis Thomas Watson the following year.
This painting is recorded in the Alfred Beit collection at the beginning of the 20th century. Having amassed a considerable fortune as a mining magnate, the philanthropist Alfred Beit (1853-1906), was able to build up a fine collection of pictures from the late 1880s under the guidance of Dr. Bode, director of the Berlin Museum, which included many of the finest examples of the Dutch and English schools.