John was educated in Ireland and became a civil engineer before emigrating to Pennsylvania.
Armstrong, who seems to have acquired early the confidence of the authorities, was commissioned captain in January 1756 and lieutenant-colonel in May, and was sent by the governor in the following year with 300 men against Kittanning. This was a town of the Delawares on the upper course of the Allegheny River, a headquarters for scalping parties.
The enterprise was completely successful; the town was taken by surprise in a night attack on September 8, 1756; and Armstrong, who had been wounded in the battle, received from Philadelphia "thanks, medal, and plate. " It was, in fact, one of the few outstanding British successes in the early part of the French and Indian War.
Henceforth Armstrong was often styled the "Hero of Kittanning. " Two years later he was the senior officer of Pennsylvania troops in the expedition under Forbes and Washington, and had the honor of raising the flag over Fort Duquesne.
He served in Pontiac's War, and for many years was judge of the court of common pleas. When the Revolution began, experienced officers were in demand.
On March 1, 1776, Armstrong was commissioned brigadier-general, and was sent to take command at Charleston, receiving his instructions from Gen. Lee. He arrived in April, and commanded the South Carolina troops at Haddrell's Point. The actual defense, however, was conducted by others. He returned to the northern army, and in the dark days which preceded the battle of Trenton he was sent on a mission to his own part of the state "to stir up the people. "
In the critical year of 1777 Armstrong, now a major-general, was influential in state affairs prior to and after the British invasion, as is evidenced from Joseph Reed's letter to Washington.
He commanded the left wing below the ford of the Brandywine, but was not actively engaged in that battle. Subsequently he held the line of the Schuylkill, in the effort to delay the advance on Philadelphia. Three weeks later, at Germantown, he led a division, about 1, 000 of Pennsylvania militia, and in the plan of the attack it was intended that he should support Generals Wayne and Sullivan, and fall upon the British left and rear; but this scheme failed of accomplishment, one of the various factors in that day's fiasco.
Armstrong resigned that same year, but was present at a council in Valley Forge in 1778, and was sent to Wyoming Valley after the massacre.
On the whole, though Armstrong's name appears frequently in the military annals of the Revolution, it does not appear that his actual achievements measured up to his reputation gained in the Seven Years' War. He was elected to the Continental Congress in 1778, and served in 1779-80, obtaining leave of absence in the latter year. He was again a delegate in 1787-1788, and died at Carlisle. His occasional letters to Washington show his views on political and other subjects.
In 1747 Armstrong married Rebecca Lyon. They had three children.