Stanwix entered the army in 1706, rose to a captain of the grenadiers in 1739, major of marines in 1741, and lieutenant-colonel in 1745, and was appointed equerry to Frederick, Prince of Wales, in 1749.
In 1754, he became deputy quartermaster-general of the forces, and on 1 January 1756 he was made colonel-commandant of the 1st battalion of the 60th or Royal American Regiment.
In 1759, Stanwix returned to Pennsylvania, built and named Fort Pitt, and surmounted the works with cannon.
On 19 June 1759, Stanwix was appointed major-general, but he was relieved by General Robert Monckton on 4 May 1760, and became lieutenant-general on 19 January 1761.
He was lost at sea while crossing from Dublin, Ireland, to Holyhead, Wales, in a packet boat The Eagle.