The sitter was the only daughter of Algernon Seymour, 7th Duke of Somerset and 1st Earl of Northumberland, and the granddaughter of Joceline, 11th Earl of Northumberland. She married Sir Hugh Smithson (1714-1786), later 2nd Earl of Northumberland, the only son of Langdale Smithson of Langdale, Yorkshire, in 1740. Her husband assumed the name and arms of the Percy by Act of Parliament in 1750, and was created Duke of Northumberland in 1766. He was also Lord Lieutenant of Northumberland (1753), Lord Lieutenant of Middlesex (1762) and Lord Lieutenant of Ireland (1763-5), was made a member of the Privy Council in 1762 and became Lord Chamberlain to Queen Charlotte.
The artist's pocket books record five appointments with the sitter in 1757, the first on 25 November, the last on 14 December. This is one of the three resulting portraits. A full-length, of similar composition, and a half-length, of oval format, are recorded in the collection of the Duke of Northumberland, the former at Syon House (see A. Graves and W.V. Cronin, A History of the Works of Sir Joshua Reynolds, P.R.A, II, London, 1899, pp. 698-9). The sitter's husband also sat to Reynolds in the same year. A full-length portrait of him in the robes of the Order of the Garter, is in the collection of the Duke of Northumberland at Alnwick Castle, and another, from a later sitting in 1762, in the Mansion House, Dublin (See A.Graves and W.V. Cronin, op.cit., pp.697-698). This portrait, which is datable to 1757/9, will be included in Dr. David Mannings' forthcoming catalogue raisonn on the artist.