A full-length portrait wearing flag officer's undress uniform, 1774-83, although the button-holes are shown grouped in threes as worn by Vice-Admirals, 1783-87. He wears a white wig, his right hand rests on a telescope and he holds his hat in his left hand. His sword is also visible by his left side. In the left background is a depiction of the relief of Gibraltar in 1781. The portrait was begun in 1783 and may not have been finished until 1786 and Darby certainly sat to Romney in 1783, 1784 and 1786. He had received his first command in 1747 and his career was uneventful until the Seven Years War, in which he served under Admiral Rodney at the capture of Martinique, 1762. In the American Revolutionary War, 1775-83, Admiral Keppel's resignation during the crisis following the Battle of Ushant in 1778 left a vacancy for command of the Channel fleet. In 1778 Darby became a Rear-Admiral and a Vice-Admiral the following year, thanks to his association with Sandwich, the First Lord, during the courts martial of Keppel and Palliser in 1779. Thus he unexpectedly came to command the Channel Fleet in 1780 at a time of grave danger for Britain. In April 1781 he relieved Gibraltar from its siege by the Spanish, for the second time during that war, and it is this event which is recorded in the portrait.

The artist was an important portrait painter of the late-18th century, generally ranked third after Joshua Reynolds and Thomas Gainsborough. He was in Paris in 1764 and in 1773 moved to Italy for two years, where he became interested in history paintings in the elevated and élitist 'Grand Manner'. This developed into improving upon nature and the pursuit of perfect form. At its best his work demonstrated refinement, sensitivity and elegance, although it could also be repetitive and monotonous. As a society painter he typified late-18th-century English artists who, compelled by the conditions of patronage to spend their time in producing portraits, could only aspire to imaginative and ideal painting. By 1780 Romney's portraits, according to Horace Walpole, were 'in great vogue' and he worked in an increasingly neo-classical style.