ohn Armstrong was born in Carlisle, Pennsylvania. He attended the Reverend McKinley's school and enrolled in Princeton College before the Revolution. He volunteered for army service in 1775. Armstrong was first an aide to General Hugh Mercer and then to General Horatio Gates during the Saratoga campaign. While with Gates at Washington's Newburgh, New York, headquarters, he anonymously penned the mutinous Newburgh Resolves. This document threatened a military insurrection if Congress continued to withhold the Continental Army's wages.

fter the war, Armstrong served as secretary to Pennsylvania's Supreme Executive Council. The council sent him in command of the militia to mediate an armed dispute over land rights between settlers and speculators in the state's Wyoming Valley. He returned from the frontier as a delegate to Congress, where he supported ratification of the proposed federal Constitution. In late 1800, Armstrong entered the U.S. Senate, where he represented New York intermittently for four years. He became America's Minister to France in 1804, and served the Jefferson and Madison administrations in negotiations for the Florida Territory.

ith the renewal of war between Britain and America in 1812, Brigadier General Armstrong took command of New York City's defenses. He became Secretary of War in 1813. During his tenure, American forces were repeatedly defeated in the Northwest Territories and Canada, and the British captured the national capital Washington. Armstrong resigned in 1814. He returned to his New York estate to write histories and biographies.