The colonies of East and West Florida had been Spanish possessions until the 1760s when they were ceded to Great Britain at the end of the Seven Years' War.

Captain Johnstone and some of the Senior Non Commissioned Officers within the company became involved in the training of gunners from the German Regiment of Waldeck, that were part of the Pensacola garrison.

The company remained in West Florida - the only unit of the Royal Artillery in the southernmost colonies - as the American Revolutionary War raged in the north.

Whilst the war had not yet spread to West Florida, the men of the company had to endure hardships, including tropical heat and a volatile relationship with the native tribes.

With forces newly arrived from New York and reinforced with soldiers from Pensacola's garrison, the army embarked upon the campaign for control of the southern colonies.

These battles included Brier Creek, Stono Ferry, the Siege of Fort Morris in Sunbury and a number of skirmishes.

[citation needed] One of the officers from the company was also appointed CRA later in the campaign, during siege of Charleston, which ended with the largest single surrender of American forces during the war.

Captain Johnstone and his company then returned to Florida, where they remained and where concerns of a Spanish attack on the colony increased following the entry of Spain into the war in 1779.

To counter this threat, Captain Johnstone received reinforcements in the form of a detachment from another company within the battalion which were placed under his command in Augusta.

in addition to large numbers from the Creek and Chickasaw tribes held out against this force for several weeks as they awaited reinforcements from Jamaica.

[citation needed] Captain Johnstone and his men were mentioned in dispatches for their bravery in rushing to a breach in the wall and bringing up a gun, holding off the Franco–Spanish forces and enabling the wounded to be carried off.

The general commanding the garrison singled out Captain Johnstone and his company on several occasions in dispatches for their bravery and energy in engaging the enemy during the siege.

After the articles of surrender were signed the company were transported by Spanish ships to the British garrison at New York where they remained until the end of the war.

Taking with him a detachment from the company as well as scientists and engineers from across Great Britain, he set out with the two small steamers Tigris and Euphrates, which he dismantled and carried across the desert to the rivers that were their namesake.

The Battery was involved in clearing Russian, American and Cuban personnel off remote Caribbean islands in the region and arrested 60 men who later turned out to be CIA operatives, causing a minor political incident.

The Battery was posted to 22 Regiment Royal Artillery a couple of years after its formation and took part in a number of operations with the Brigade.