Born in Sydney, Johnston grew up in the early colonial days of New South Wales and entered the British Navy at a young age, and was the first Native-Born Australian to do so.

His Father was distinguished Lieutenant-Colonel George Johnston of the New South Wales Corps, or 102nd Regiment, who was the chief actor in the deposition of Governor Bligh.

Returning to England after having performed this duty Malabar was paid off, and young Johnston joined the Namure, receiving ship, as midshipman, and shortly afterwards was appointed to Semiramis, a 36-ton frigate, commissioned for active service off the coasts of Spain and Portugal.

[3] Johnston and another officer were subsequently placed with a crew of 150 men on board the captured French 80-gun ship Neptune — one of the vessels that escaped from Nelson's fleet after the Battle of Trafalgar — and ordered to proceed with her to Majorca, where she was to be laid up; and this service having been accomplished, he returned to Norge, which shortly afterwards sailed to England for the purpose of joining the Scheldt fleet under the command of Admiral Sir William Young.

At this period Great Britain was at war with the United States, and the Asia sailed for Bermuda, where Johnston received his commission as lieutenant, and was appointed to the command of an advice vessel, or what at the present day would be called a despatch boat.

This led to his being engaged in the service of landing troops for an attack on New Orleans, against which place and Mobile a large expedition was sent, and, after the repulse of the British, in bringing the wounded from the front.

Peace was shortly afterwards proclaimed, and Lieutenant Johnston receiving promotion, was appointed second-lieutenant of his old ship, Asia, which was then under the command of Captain Alexander Skeene, and was employed cruising off the coast of West Florida.

While engaged in this duty, his party were the first settlers to discover a river—known to the Walbanga local people as Bhundoo—that he named the Clyde, and, on returning to Sydney, accompanied the Governor on a visit of inspection to Newcastle and Port Macquarie.

[3] Robert Johnston died of old age with a funeral taking place at Annandale on a Sunday afternoon, which was attended by about 150 friends and family, and viewed by a large gathering of people from the surrounding neighbourhood, who watched the proceedings with respectful interest.