Stanwix joined the Army and had become a captain-lieutenant in Hasting's Foot Regiment by 1692.[1] In March 1702 he was elected Member of Parliament for Carlisle.[2] He was appointed Lieutenant-Governor for Carlisle in 1705 and found that his main challenge was stopping the smuggling across the border between England and Scotland.[1]
In 1703, during the War of the Spanish Succession, Stanwix was present at the Battle of Caia in Portugal.[1] In 1711 he became Governor of Gibraltar.[1] Except from a personal perspective he was unsuccessful as a governor, as his main achievement was to become richer than when he arrived. Observers felt that he should have concentrated on encouraging the Dutch to leave so that the benefits of the Capture of Gibraltar (in 1704) could be directed entirely in Britain's direction. Stanwick was tenacious as even when he was replaced by David Colyear he stayed on as lieutenant-governor for some months.[1]
Stanwix died in 1725.[1] He left his estates in Carlisle and Middlesex to his wife, and on her death to his nephew John Roos, on condition that he assumed the surname of Stanwix.[5]