St Aubyn was born on 27 September 1696, the eldest son of John St. Aubyn, 2nd Baronet and his wife Mary de la Hay, daughter and coheiress of Peter de la Hay of Westminster.

Joining the opposition against Robert Walpole, he was hostile to the Septennial Act, and the employment of Hanoverian troops with the standing army.

On 9 March 1742, after Walpole's fall from power, he seconded Lord Limerick's motion for a committee to inquire into the transactions of the previous two decades, which was defeated by 244 votes to 242.

A fortnight later he seconded a motion by Limerick for a secret committee of 21 to examine Walpole's official acts during the last ten years, and it was carried by 252 votes to 245.

[4] Jacobite sympathies were at one remove: he briefed Thomas Carte on parliamentary debates, for the benefit of the Old Pretender, who gained an exaggerated view of St. Aubyn's effective support.