In 1738, he persuaded his father to send him to Saint Petersburg, where he stayed with his mother's uncle, James Hewitt, an expatriate Scot and merchant. Another uncle, Gustav Otto Douglas (1687-1771), was a Russian general, who was born in Sweden, then changed sides after being captured at Poltava in 1709. Douglas arranged a commission for Johnstone in the Russian military but his father threatened to disinherit him if he accepted and after a period in London, he returned to Edinburgh in 1740.