(Updated, May 2024) A half-length portrait to left in the 1767-74 captain’s uniform. Although bald, Johnstone does not wear a wig and is shown making what may be a characteristic conversational gesture of emphasis with his right forefinger on his left thumb. This portrait is either from a recorded oil by Henry Raeburn, of which a better copy was sold by Bonhams, Edinburgh, on 11 October 2023 (Scottish Art, lot 3) or from an earlier miniature by John Bogle - also a Scot, as was Johnstone - of which three versions are known. One of two bearing Bogle's signature to left of the figure has been in the Scottish National Portrait Gallery since 1982; the second was last seen at sale by Christie’s, London, in 1986; the third, signed to the right, is at time of writing (May 2024) with Philip Mould & Co., London. As its current cataloguing there observes, it appears that Raeburn’s portrait was a posthumous one and - unusually for an oil artist - based on one of the miniatures, not the other way round. The reason is that Raeburn only painted Johnstone’s widow Charlotte in 1791, shown holding one of the Bogle miniatures, and would himself have only been 18 at most when Johnstone last wore the uniform shown in 1774. Bogle, born about 1744, would then have been about 30 and was already exhibiting his work, including at the Royal Academy, 1772-94. He showed just one miniature titled 'Portrait of a navy officer' there in 1785, which is the approximate date usually given to those of Johnstone on the assumption that it was one of them. If it was, the prime version must still have been painted before 1774 owing to the uniform pattern, but it could have been of someone else, especially as only one of Bogle’s 23 submissions there - some in groups - identifies any of his sitters by name (‘the late Dr W[illiam]. Hunter’, also in 1785). It should be noted, however, that both the SNPG miniature and that sighted with Philip Mould (2024), and the oil sold by Bonham's in 2023,show Johnstone probably sitting with his left elbow resting on the corner of a table covered in green baize. This oil shows him standing, no table, and is the only version yet sighted with the swag and tassel in the top right corner and the architectural line down the edge below..

Johnstone was an aggressive personality who reportedly duelled with one of his early superiors and wounded him. He is also said to have killed a captain’s clerk in a duel ten years later, and faced court-martial for insubordination. His bravery in various minor actions saw him survive these setbacks and in 1763 he was appointed governor of West Florida, a role he fulfilled with some success in the early years but less so later.
In 1768 he returned home and went into Parliament where, according to his original Dictionary of National Biography entry by Sir John Knox Laughton, 'his total want of fear and his adroitness with a pistol made him a useful addition to his party’. After a further brief diplomatic posting to America in 1778, Johnstone returned to sea in 1779, as a Commodore (a temporary rank for a senior captain, usually when acting as a squadron commander). In 1781 he was given command of an expedition to take the Cape of Good Hope from the Dutch but was surprised en route, while anchored at Porto Praya in the Cape Verde Islands, by an eastward-bound French squadron alerted to the plan and commanded by the formidable Admiral de Suffren. In the circumstances Johnstone escaped lightly but Suffren reached the Cape before him and foiled his intended attack by removing all element of surprise.

Johnstone somewhat retrieved the situation by a bold assault on six Dutch East Indiamen anchored in Saldahna Bay, of which one blew up and he captured the other five before returning home, marrying Charlotte Dee at Lisbon on the way in 1782. His later life was as an MP in the interest of the East India Company, of which he was a director, and one of legal dispute only resolved just before his death. This occurred while taking the waters at Hotwells, Bristol, and probably from Hodgkins' disease, of whch the onset probably affected his judgement. Although undoubtedly brave, Laughton’s late-Victorian view was that he lacked 'temper and restraint' but his entry in the modern Oxford Dictionary of National Biography (by Robin Fabel) is more considered. Without extenuating his faults it also casts doubt on his reputed history as a duellist by making no mention of it.