Following the capture of Lieutenant-Governor General Henry Hamilton in 1779, DePeyster became the de facto military leader of British and Indigenous forces in the Ohio Country and the upper Great Lakes region.

[3] In 1745, Shirley, along with de Peyster's uncle, Col. Peter Schuyler, had directed the Siege of Louisbourg against the French in today’s Nova Scotia.

De Peyster next held a commission in the 51st Foot, a regiment raised by Lieutenant General Robert Napier in America, which at one point, had three Schuylers in it.

[4] During the French and Indian War, he served under his uncle in the Province of New York, gaining experience in frontier warfare.

[1] Until the summer of 1777, British policy during the American War of Independence was for their Indigenous allies to be ready to support the Crown but remain inactive.

[5] DePeyster, by his tact and the adoption of conciliatory measures, effectively managed Britain's Indigenous allies against American militia from Pennsylvania and Kentucky.

[7][1] He retired in 1794 due to illness, and sold his lieutenant-colonelcy to an associate of John Fane, 10th Earl of Westmorland, the then Lord Lieutenant of Ireland.