Johnstone went to sea as a boy, in the merchant service, and subsequently entered the Royal Navy. He served with distinction during the war of the Austrian succession and the seven years’ war, and in 1763 was placed on half pay. In November, at Bute’s recommendation, he was appointed governor of West Florida.
Johnstone now became connected with Sir James Lowther, Bute’s son-in-law. At the general election of 1768 he stood as Lowther’s candidate at Carlisle and was defeated, but shortly afterwards was returned for Lowther’s pocket borough, Cockermouth. In the House he quickly displayed his energy and ability, and became a frequent and effective speaker. At first he seems to have been with Administration: he is listed as having voted with them on the expulsion of Wilkes, 3 Feb. 1769; but he voted with the Opposition on the Middlesex election in both 1769 and 1770. It is extraordinary that Lowther, who at this time was a Government supporter, should have allowed Johnstone to take an independent line in Parliament, a privilege he never accorded to any other of his Members. But Johnstone did not vote with the Opposition on the Spanish convention, 13 Feb. 1771; on the royal marriage bill, March 1772, was classed by Robinson as merely ‘doubtful’; and when he voted for making the Grenville Act permanent, 25 Feb. 1774, was listed as one who normally supported Government. He took an active part in the politics of East India House, where he opposed any suggestion of Government interference in the affairs of the Company,
North’s American policy drove Johnstone back into opposition, and he was one of the few who spoke against the Boston port bill.
The decision of the Rockinghams to support the recognition of American independence, and the approach of war with France, brought about a radical change in Johnstone’s political position. In February 1778 he approached the Government, through Wedderburn, about a command at sea.
He was extremely sorry to see the idea adopted by gentlemen with whom he had acted; that if he found he had been acting with gentlemen who were ready to give up the supremacy of this country over America ... he would sooner cross the floor and join those whose measures he had always disapproved.
He now accepted a place on the Carlisle peace commission to America, a step which aroused much ridicule after Johnstone’s earlier criticism of Carlisle’s appointment.
Before leaving, Johnstone re-affirmed his opposition to independence: he thought the repeal of the Quebec Act and the Declaratory Act would be sufficient to reconcile America.
Whatever the people in power may wish and aim at, the great body of the people do not wish to change the government of Britain for that of Congress. The people of old settled interest and property do not wish for independency, they rather dread it.
He hoped to negotiate a settlement which would preserve some form of Anglo-American union. In America Johnstone tried to supplement the formal negotiations between the commission and Congress by private ones with individual American leaders, and on their failure returned home in advance of the other commissioners.
Lord Chatham’s death I consider as the greatest evil that has yet befallen us. My ideas are the same as he held: do ample justice to all your fellow citizens but maintain your own indisputable rights ... The ill success of Howe and Keppel (who were my darlings as superior sea officers) against an equal if not inferior force confounds all my sense and sickens my very soul. Something must be done. We cannot submit that France shall dictate the terms on which this nation shall hold her dominions. It is a vain thought to imagine that granting the unjust claim of independence to the united colonies of America, that this will dissolve the power of our enemies or separate France and America. We shall lose a great deal and gain nothing but contempt by such a proceeding. Whereas by perseverance, if there was any head to direct or to execute the most obvious operations, our success seems certain.
Johnstone now opened negotiations with Government. He was very bitter against Lord Howe, whom he blamed for the failure of naval operations in America; ‘thought Lord George Germain and Lord Sandwich must go out’; and offered a tempting prospect of substantial gains from the Opposition.
He did nothing without the consent of Sir James Lowther, who would also in that case accept some small trifling office to mark his connexion; that he thought he could bring Charles Fox in, and that the Duke of Grafton and all his friends would come in with Lord Camden and the Grenvilles; and he rather thought also the Shelburnes might.
North may well have been sceptical of Johnstone’s ability to bring over to Government all these members of the Opposition; in the event none of them came. Johnstone received no political office, but was appointed to the command of a ship with a promise of a bigger command later.
At the general election of 1780 Lowther left out Johnstone in the first arrangement of his boroughs, and Lord Loughborough urged John Robinson to provide him with a Government seat.
It would be idle to enlarge upon the advantage of bringing in so powerful and active a friend, who you know does nothing feebly and would be most warmly attached to Lord North if he felt an obligation to him. I have reason to think that Sir James still intends to offer him a seat upon the second cast of his Members, which in his present disposition Johnstone will refuse, but I hope and trust that I am not too late in suggesting to you to anticipate the reconciliation which would probably take place between them.
The advice was acted upon, and Johnstone was found a seat at Lostwithiel. During the next eighteen months he was mostly absent at sea. Early in 1781 he was given command of an expedition to the Cape of Good Hope, but failed to reach the Cape before the arrival of French reinforcements had made his undertaking impracticable. He returned to England in time to vote with North’s Administration in the last two divisions before its fall.
In May 1782 Johnstone criticized the Rockingham Administration for their recall of Rodney, and praised Rodney’s unorthodox tactics in breaking the line at the battle of the Saints.
Wraxall wrote about Johnstone:
Nature had cast his person in a coarse but vigorous mould ... irascible, intemperate, violent, he was a warm and zealous friend but an implacable enemy. He possessed a species of ardent, impetuous, half-savage eloquence, restrained by no delicacy of language, yet capable of powerfully affecting his hearers by the display of information, by his energetic appeal to their passions, and even by his gesticulations.
His generosity and lack of rancour is indicated by the fact that, though a Scot, he lent money to Wilkes—and was apparently never repaid.