Thomas Robinson (1738-1786), the elder son of Thomas, 1st Lord Grantham, and his wife Frances, daughter of Thomas Worsley of Hovingham, Yorkshire, was born in Vienna where his father was Ambassador in 1730-1748. After Westminster and Christ's College, Cambridge, he was elected Member of Parliament for Christchurch. At the Congress of Augsburg, in April 1761, he was appointed a Secretary of the British Embassy. In February 1770 he was appointed Vice-Chamberlain of the Household, and became a Privy Councillor; in September he succeeded his father as Baron Grantham. The following year he was made Ambassador at Madrid, but retired under a cloud a few years later. He subsequently became first commissioner of the Board of Trade and Foreign Plantations, and then Secretary of State for Foreign Affairs, assisting Shelburne with peace negotiations of 1783 with France, Spain and America, and with the treaties Britain first acknowledged the independence of the United States.
On 17 August 1780 he married Lady Mary Jemima Grey Yorke, younger daughter of Philip, 2nd Earl of Hardwicke; they had two sons. He was succeeded by the eldest, Thomas Philip, who also succeeded to the earldom of De Grey.
Sittings are recorded in March, April and May 1760, and two portraits were painted: one recorded as being in the family and then in the collection of Baroness Lucas, and the present painting, which was probably painted as a gift for the Duke of Newcastle, who was responsible for Grantham's father's enoblement. Romney painted a portrait of Grantham in 1781 with the Escorial Palace in the background.