St Clair became an ensign of 6th Regiment of Foot in 1694, however was set on halfpay in 1713.[3] In the next year, he was admitted to the 3rd Foot Guards and was promoted to captain in 1714.[3] He served as 2nd major of his regiment from 1722 and as 1st major from 1725, having been advanced to the rank of brevet colonel two years before.[3] In 1734, St Clair was appointed to the command of the 22nd Regiment of Foot[3] and three years later he was transferred to the colonelship of the Royal Regiment of Foot (later renamed as 1st (Royal) Regiment of Foot), which he held until his death.[4]
He rose to brigadier general in 1739 and already after another two years to major general.[5] St Clair became lieutenant-general in charge of the British forces in Flanders in June 1745[6] and in the following year, he was sent with six thousand men to attack Quebec.[5] Because of delays, he sailed instead to capture the Breton port of Lorient.[5] He destroyed the French fortifications near Quiberon and then returned to England.[5] In 1761, St Clair was finally advanced to the rank of full general.[5]
In 1735, St Clair bought Rosslyn Castle, which was later inherited by the male heirs of his sisters.[8] On the death of his older brother John St Clair in 1750, he succeeded de jure as Lord Sinclair, but never assumed the title, preferring to retain his seat in the Commons.[9]
^ abcBurke, John (1832). A Genealogical and Heraldic History of the Peerage and Baronetage of the British Empire. Vol. I (4th ed.). London: Henry Colburn and Richard Bentley. p. 441.