John Armstrong was born in March 1674, the eldest son of Robert and Lydia Armstrong of King's County (County Offaly), Ireland. He began his military career at an early age, taken prisoner and sent to Limerick in 1691. Later in the 1690s he served in Flanders, after which the Duke of Ormond appointed him to join the second troop of Horse Guards. Under Ormond's leadership, Armstrong received tutoring in mathematics and engineering.

Armstrong returned to Flanders in 1703 and so impressed colonel of artillery and chief engineer Brigadier Holcroft Blood that Blood introduced Armstrong to the Duke of Marlborough. Armstrong quickly earned Malborough's respect and gratitude and in the following years was promoted until reaching the rank of colonel in 1712, with an appointment to quartermaster-general of the British Army. After the Treaty of Utrecht, Armstrong served as one of the commissioners overseeing the demlotion of the harbor and fortifications at Dunkirk, and in 1714 after the accession of George I to the British throne, Armstrong was appointed chief engineer of England. He was briefly in command of a regiment of foot, previously under the command of Brigadier-General Stanwix, but the regiment was disbanded within a year. He did not receive another command until 1735, when he was given the colonelcy of the Royal Regiment of Ireland (18th foot). By 1739, he had risen to the rank of major-general.

Armstrong was made surveyor general of ordnance in 1722 and admitted as a fellow of the Royal Society in 1723. Throughout the 1720s into the 1730s, he made trips to Paris to aid in negotiations with the French, Dutch, and Spanish, and he was considered the expert on Dunkirk's harbor and fortifications. He had made an enemy in the Duke of Argyll, who took over British military matters from the Duke of Marlborough.

In 1714, Armstrong married Anna Priscilla Burroughs (1688/1689-1725), with whom he had five daughters. Anna died giving birth to their last daughter. John Armstrong died in 1742 and was buried in the Tower of London's chapel.

[Information retrieved from Latcham, Paul, "Armstrong, John," Oxford Dictionary of National Biography, 2014.]