Princess Mary (1723-72) was the seventh child and fourth daughter of George II and Caroline of Ansbach. After her mother's death in 1737, Mary was brought up by her elder sister, Princess Caroline. In 1740, she was described as 'a lover of reading and far more solicitous to improve the mind than to adorn the body'. Horace Walpole, who had played with her when they were both children, said she had 'the softest, mildest temper in the world'. When she was 17, a marriage was arranged for her with Prince Friedrich of Hesse-Cassel (1720–85) and they had four children but her married life was very unhappy. Prince Friedrich converted to the Roman Catholic faith and this gave Mary the excuse to separate from him in February 1755. Mary did not return to England but stayed with her children, supported by her father-in-law, Wilhelm VIII, who provided a residence for her at Hanau. She died at Hanau on 14 January 1772.

In the miniature the red pigment has faded from Mary's lower lip. The portrait is either a fake or a copy based on an eighteenth-century portrait to which a false monogram has been added. The Hatfield case is engraved with her name and 'copied from a Miniature (by Ozias Humphry) at Gotha'. A photograph of this, however, shows it to be slightly different from the original. The photograph is pasted into an album of engravings in the Royal Library and annotated by Queen Mary: 'Mary, daughter of King George II & wife of Frederick II, Landgrave of Hesse. My great great grandmother'.

Ozias Humphry (1742-1810) was born in Devonshire, moved to Bath where he had rooms in the house of Thomas Linley, the musician, and then went to London where, in 1764, Sir Joshua Reynolds persuaded him to settle. Humphry spent a brief period in India (1785-7), exhibited at the Royal Academy from 1779 to 1797 and became a Royal Academician in 1791. His sight began to fail in 1792 when he turned to drawing in oil and pastel. He became blind in about 1797-8.

Crudely inscribed with the initials OH, a falsification of Ozias Humphry's monogram.