A three-quarter-length portrait to right in captain's, over three years, undress uniform, 1748-67 and a tie wig. The empty right sleeve of his coat is pinned across and his hat is held in his left hand. It is inscribed in gold, top left, 'Captain Frederick Cornewall R.N.'

In 1744 as a lieutenant in the 'Marlborough', 90 guns, commanded by his cousin James Cornewall, the sitter fought in Mathews's inconclusive action off Toulon, where he lost his right arm. His cousin was killed in the fighting and Cornewall was made post-captain immediately afterwards. Although he was appointed to the vacant command his health prevented him from executing his duty. At Admiral Byng's action off Minorca in 1756 he commanded 'Revenge', 70 guns, and it was his evidence at the court-martial which was largely responsible for the guilty verdict that resulted in Byng being shot. He does not seem to have had any further sea service. Cornewall was MP for the Montgomery Boroughs, 1771–74.

This portrait was painted in Bath, where Gainsborough lived and worked between 1759 and 1774. As the sitter and his wife are recorded in Bath in March 1762, this provides a convincing date for the portrait. The painting is among the artist’s best works from this period. It was bequeathed to the museum in November 1960 and was for many years miscatalogued as ‘British School, 18th century’.