Oval miniature in watercolour on paper in an oval gilt metal suspension frame, with a glazed back containing hair with an overlaid gilt monogram, GAD or GASD. The sitter is shown short bust-length, facing forward, but looking slightly to his right, wearing flag-officer's undress uniform c. 1774-83 and a short white wig.

Darby is said to have been the second son of Jonathan Darby III (d. 1742/3) of Leap Castle, King's Co., Ireland. He became a naval lieutenant in 1742 and a captain in 1747. As such, he saw considerable active service in the Seven Years War, 1756-63. He became a vice-admiral in March 1779, when the Navy was convulsed by both the political consequences of the inconclusive Battle of Ushant of 1778, after the French entered the American War, and the soon-fulfilled threat that the Dutch would join France and Spain as the American rebels' allies. Political considerations meant that a number of senior admirals refused to serve at the time and, despite lack of active fleet experience, Darby was appointed commander of the Channel Fleet late in 1780. He proved a model of steadiness, resolution and tact, and soon became both liked and respected in this principal pillar of national defence. Over the next two years he competently handled a number of difficult situations at a time that the Navy was particularly weak, though (partly by chance) without any fleet action against what would usually have been superior odds. These included commanding the second relief of Gibraltar from Spanish siege in 1781, and effective dispositions to defend the Channel against potential incursion by a Franco-Spanish fleet of 40 ships to his 21 later that year. Victorian denigration of him for not actively seeking and / or finding combat, whatever the odds, is therefore ill-judged. When Lord North's administration fell in 1782, taking Lord Sandwich with it as First Lord of the Admiralty, he resigned and retired. In poor health, he died within three weeks after his second wife in November 1790.

Darby had two sons by his first wife, Mary St Quintin (d.1773), who was daughter of Sir William St Quintin, 4th baronet, of Scampston Hall, Yorks, Sheriff of Yorkshire and MP for Thirsk. Her brother William, 5th baronet, died without issue in 1795, his heir being Darby's eldest son William Thomas, who on inheritance changed his surname to St Quintin. The second son, Matthew Chitty Darby (1772-1823) was a major-general in the army, and called Darby-Griffith from 1801. The first name of Darby's second wife (née Jackson and widow of Thomas Bridges), whom he married before 1777, does not seem to be known. Assuming the 'A' or 'AS' in the initials 'GAD' here are not hers, they are presumably his, though what they stand for is also unknown. The Museum also has another miniature (MNT0182) and a fine full-length portait of Darby by Romney (BHC2643), possibly slightly later than this modest-quality image, but the likenesses are a good match.