Although unsigned, this miniature portrait has been attributed to John Donaldson (1737-1801), a Scottish artist who also worked in London.
One of the bracelet clips at the rear is engraved "Rt Hon'ble William Pitt" for William Pitt (28 May 1759 – 23 January 1806) the famous British Prime Minister of the late 18C. He was the son of William Pitt the Elder (1708-1778), who was Prime Minister of Great Britain (1766-1768).
William Pitt the Younger became the youngest ever Prime Minister of Great Britain between (1783-1801) and again between (1804-1806).
It appears from the clothing and wig style that Donaldson painted the portrait in the mid 1780's and has slightly emphasized Pitt's youth, as he has given him softer features than the 1787 Gainsborough portrait of Pitt.
Comparison can be made with three other large oil portraits of William Pitt the Younger, all painted around the period 1785-1790 which are illustrated here. The one on the right, painted by Thomas Gainsborough c1787, appears in William Hague's biography of William Pitt.
Thus the miniature was probably painted in 1784 or 1785, shortly after the time Pitt became Prime Minister in 1783.
Pitt never married and it seems possible that the miniature was painted at the request on his mother, who must have hoped he would marry.
In 1783 he was reputedly offered as a wife, the seventeen year old daughter of the fabulously rich French politician Jacques Necker, and much later in 1796 there was some expectation that he would marry Lady Eleanor Eden, the attractive daughter of Lord Eden.
The miniature is believed to be genuine, although it is difficult to be 100% sure. However, it should theoretically be possible to confirm or disprove the identity of William Pitt as the sitter, by taking a DNA sample from the lock of hair contained in the reverse of the locket.
The locket case with the hair back is similar to, but less elaborate than one of similar date on page 325 of Foskett. The engraved bracelet clips on this miniature appear to have been added slightly later. This was not uncommon and there are various miniatures in this collection that had bracelet clips or brooch pins added after the date of the original miniature.
For much more about William Pitt, see William Pitt the Younger - Wikipedia, the free encyclopedia
For a second miniature of William Pitt in this collection see View 1135