De Peyster joined the British Army in 1755, joining the 50th Foot Regiment, had been raised in America in 1748, by William Shirley, the Governor of the Province of Massachusetts Bay. In 1745, Shirley, along with de Peyster's uncle, Col. Peter Schuyler, had directed the Siege of Louisbourg. Next, he held a commission in the 51st Foot, a regiment formed by Lieutenant General Robert Napier in America, which at one point, had three Schuylers in it. During the Seven Years' War, he served under his uncle in upper colonial New York, gaining experience at frontier American warfare. He was captured, held as prisoner in France, and served out the war with the 8th Regiment of Foot in Germany after being exchanged. The 8th Regiment was assigned to Canada, and DePeyster enjoyed a series of promotions. In 1774, he was appointed commandant of Fort Michilimackinac, in present dayMackinaw, Michigan. DePeyster spent the next five years at the Fort.
American Revolution
When war broke out with the United States on April 19, 1775, DePeyster recruited Native Americans from the Great Lakes region to serve the British Crown, notably the effort under General John Burgoyne in his native colony of New York. He was rewarded with a promotion to major. In 1779, Major DePeyster took control of Detroit. The American Indian tribes of the Northwest Territory were then hostile to the British, but DePeyster, by his tact and the adoption of conciliatory measures, entirely weaned them from the colonists, and effectively managed his American Indian allies against American militia from Pennsylvania and Kentucky. Although Great Britain ceded control of Detroit to the United States at the end of the war, Detroit remained in British control until 1796. In November 1783, DePeyster was informed that he was promoted to Lieutenant-Colonel and was being transferred to Fort Niagara, situated at the connection of the Niagara River and Lake Ontario. However, he did not leave for Niagara until 30 May 1784, where he assumed command on 5 June 1784. In the summer of 1785, after the wars completion, he set sail and returned to England with his Regiment and continued to serve, eventually receiving a commission as colonel, on 12 October 1793.
Later life
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, who had failed to pay for it ten years later. After his retirement, DePeyster and his wife moved to Dumfries, where they settled down at Mavis Grove, a country estate. In 1795, when England was threatened by Napoleon, he again became actively involved with the militia. He had a large share in enlisting and drilling the 1st regiment of Dumfries volunteers, one of the original members of which was Robert Burns, the prominent Scottish poet, who dedicated to him his poem on "Life," and with whom he once carried on a poetical controversy in the columns of the Dumfries Journal. DePeyster also published Miscellanies, by an officer in 1813.
Personal life
After the Seven Years' War, he was stationed in Scotland, where he married Rebecca Blair, a daughter of Robert Blair, Provost of Dumfries, and sister-in-law to Lieutenant-Colonel Bryce McMurdo. They married in 1757, and purportedly had a happy, but childless, marriage and were seldom apart. De Peyster died as the result of an accident on 26 November 1822 in Dumfries, Scotland. A large funeral was given in his honor, and he was buried in St Michael's Churchyard. His wife died on 20 February 1827.