St George was the son of Sir Oliver St George, 1st Baronet and Olivia Beresford, daughter of Michael Beresford, of Coleraine, County Londonderry.
In 1689 he took command of Solomon Richards' infantry regiment which had just taken part in a failed expedition under John Cunningham to rescue the besieged city of Derry, the failure of which led to the dismissal of both Cunningham and Richards.
Under St George, the regiment took part in the more successful relief operation of General Percy Kirke in July 1689.
St George died in August 1735 when the baronetcy and barony became extinct.
One of his daughters, the Honourable, Mary St George, married John Ussher.