 St Clair was the second son of Henry St Clair, 10th Lord Sinclair and his wife Grizel Cockburn, daughter of Sir James Cockburn, 1st Baronet.As a child he received a commission into the 1st Battalion, Royal Regiment of Foot.St Clair became an ensign of 6th Regiment of Foot in 1694, however was set on halfpay in 1713.