Career
Armstrong joined the Williamite Army of Ireland in 1691. In 1697 he joined the Duke of Ormonde"s Horse Guards. In August 1711, during the War of the Spanish Succession, he constructed overnight a large battery of artillery with which the enemy was bombarded furiously and successfully in the morning at the Siege of Bouchain.
He was appointed Quartermaster-General to the Forces in 1712 in recognition of his performance at Bouchain.
After the Treaty of Utrecht, in Spring 1713, he was appointed one of the Commissioners responsible for overseeing the dismantling of the harbour at Dunkirk. In 1714, following the accession of George I, he was appointed Chief Engineer.
He recommended the split in 1716 of the Ordnance Service into the Royal Engineers and the Royal Artillery. He was made a fellow of the Royal Society in 1723.