This drawing was copied after one of Thomas Gainsborough's (1727 - 1788) many portraits of William Pitt the Younger (1759 - 1806). The first version was painted by Gainsborough in 1787. Pitt became the youngest Prime Minister ever of the United Kingdom in 1783 when he was just 24. He left office in 1801, then became Prime Minister again from 1804 until his death in 1806.
This drawing is believed to be a copy of the Gainsborough portrait of Pitt in the Paul Mellon collection, housed at the Yale Centre for British Art in the United States (acc. number B1981.25.300).