St Clair was born in Gibraltar in 1785 where his father, William, was a colonel of the "Scottish Borderers" - the 25th regiment.

His father was a friend of Prince Edward, Duke of Kent and Strathearn and he was able to use this to advance his career more rapidly.

[2] He signed up for the British Army, joining the 94th Regiment of Foot in 1803, and the following year he was made a lieutenant.

In 1806 he was posted to the West Indies as an ensign in the Royal Scots, who garrisoned Berbice and Demerara during the first decade of the 1800s.

[2] This included an account of his journey to Stabroek aboard HMS Brilliant that arrived early in 1806 and left him in South America until his return in June 1808.