 In the next year, he was admitted to the 3rd Foot Guards and was promoted to captain in 1714.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.In 1734, St Clair was appointed to the command of the 22nd Regiment of Foot 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.