'Sir Watkin Williams-Wynn and Henrietta, his wife' depicts Sir Watkin Williams-Wynn, at the age of twenty, with his wife Lady Henrietta Somerset. The couple were married on 11 April 1769 and she died on 24 July of the same year. Henrietta Somerset was a daughter of his father's friend and political ally, the 4th Duke of Beaufort.

The couple are depicted life-size in matching black and pink Van Dyck costume, and are holding theatrical masks in a curtained architectural setting, beside a massive vase. The picture was presumably begun as a marriage portrait, although it is unlikely to have been completed prior to Lady Henrietta's death on 24 July 1769. Williams-Wynn sat for Reynolds in February 1769, his fiancée in March, and Sir Watkin once again in August. The black costume of the figures suggest that it was completed as a memorial portrait after Henrietta Williams-Wynn's death.

Lady Henrietta is posed in an attitude common in Reynolds' portraits of women, which derives ultimately from the studio practice of his master Thomas Hudson. Sir Watkin's attitude combines two classical prototypes: a melancholy figure from a sarcophagus of the Muses, engraved in 1747 and now in the Louvre, and the 'Farnese Hercules', in Rome until 1787, and now in Naples. In his 'Discourses', Reynolds praised the Hercules as one of three ideal types of male beauty ('muscular strength' as opposed to 'activity' and 'delicacy'). It was certainly the type most in tune with Sir Watkin's short and stout build. The vase in the background is of a type which appears in a number of Reynolds' portraits, and is copied from a seventeenth-century engraving by G.B. Galestruzzi after Polidoro da Caravaggio.

The sitters are wearing 'Vandyck' costume, which was fashionable from the 1740s and continued to appear in British portraits through to the 1770s. Sir Watkin was passionately fond of the theatre, and there are a number of references to masquerade dresses in the Williams-Wynn accounts.