Johnston was born on 19 March 1764 at
Annan,
Dumfriesshire,
Scotland, the son of Marine Lieut David Johnston, who during the American Revolution, was Marine Captain-Lieut and
aide-de-camp to Lord Percy, later the
2nd Duke of Northumberland. It may have been Percy who obtained a commission for the 12-year-old Johnston as
second lieutenant of
marines on 6 March 1776. Johnston went to America with the 2nd Marine Btn, and took part in the
American War of Independence, serving in New York and Halifax during 1777 and 1778, after which he was promoted on 27 April 1778 to
first lieutenant in the 91st Plymouth Marines. During the campaign, his father was wounded but survived and went on to serve in the Marines with his final record as Major appearing in March 1784. The Duke of Northumberland, who had held Johnston's father in high regard, became Johnston's sponsor. The young officer subsequently acted as a recruiting officer in 1779 and 1780 in the United Kingdom before serving aboard in the East Indies in 1781, and suffering a severe wound in action against the French. Returned to garrison duty in
Portsmouth, he volunteered to join the
New South Wales Marine Corps, which would accompany the
First Fleet to
New South Wales. He sailed for Australia aboard the convict transport
Lady Penrhyn in 1787. ==New South Wales==