Armstrong was born in 1780 at
Ballynahinch, County Down. He was the son of John Armstrong, who married a daughter of Rev. John Strong, for 36 years (1744–1780) presbyterian minister of Ballynahinch. He was a descendant of John Livingstone, of
Killinchy, one of the founders of
Irish presbyterianism. He was first trained at the
Rademon Academy, under
Moses Neilson, after which he became classical assistant to
William Bruce at
Belfast Academy, and conducted a special class of sacred history. He graduated at
Trinity College, Dublin, and studied philosophy in the
University of Edinburgh under
Dugald Stewart. He was licensed 11 May 1806 by the
Presbytery of Antrim (non-subscribing). The same year he received calls to Clonmel and Strand Street, Dublin (2 October); choosing the latter, he was ordained 25 December 1806 by
Dublin Presbytery (non-subscribing) as a colleague to John Moody,(b. 11 Dec. 1742, d. 15 July 1813), after whose death
William Hamilton Drummond, became (25 December 1815) his colleague. He was one of the founders of the
Irish Unitarian Society (1830) and of the
Association of Irish Nonsubscribing Presbyterians (1835), and he represented the latter body at the celebration of the tercentenary of the reformation at Geneva in August 1835. In the previous year he had received the degree of DD from the
university of Geneva. He was a member of the
Royal Irish Academy. He died very suddenly at Stonehouse, Stillorgan Road, Dublin on Wednesday, 4 December 1839, having preached on the previous Sunday, and married a couple that very morning. He married Mary Allman, and left two sons (John Strong Armstrong, A.B., president of the
Dublin Historical Society, and Rev. George Allman Armstrong, A.B., originally a barrister, who succeeded him in 1841 at Strand Street) and four daughters. A petition from his widow is printed in
Parliamentary Debates on the
Dissenters' Chapels Bill, 1844. ==Works==