Johnstone was four when his father died. According to his aunt Miss Dee, ‘Jackey’ was ‘stout and well and merry; happily for him he is too young to know the magnitude of his loss’. Sure that his only legitimate son would ultimately inherit Westerhall from his childless uncle William, Johnstone’s father made substantial provision for his natural sons in India and left his lawful heir £200 per annum, with the injunction that from the age of six he should be educated in Scotland as befitted the heir to an ancient estate in Dumfriesshire. At Edinburgh ‘Jock’ prepared for university with a tutor.
In 1800 he became an ensign in the Coldstream Guards. In London he got in with a ‘bad set’. In February 1802 when he was ill at Edinburgh, the Duke of Cumberland took a keen interest in his welfare and induced Richard Vyse to make him his aide-de-camp. He got into a scrape and in June 1802, when he was ordered to his regiment in London, left debts of over £10,000 in Edinburgh. In London he behaved no better, associating with blackguards and absconding to Scotland. The duke washed his hands of Johnstone, who promised to mend his ways and for a time resumed his military career.
A year after his marriage he succeeded to his uncle William’s baronetcy and Dumfriesshire estate. On the strength of this he gave up the army and contested the ensuing county election (1806), ‘vigorously’ supported by Lord Grenville whose administration he backed; but he was defeated. He then hoped to come in for Dumfries Burghs, or to purchase an English borough. In 1807, after promising to try the county again, he offered himself instead at Taunton, but withdrew.