[3][4] Armstrong was educated at the Dragon School and then at Eton College, where he was a King's Scholar, following which he went up to Christ Church, Oxford, where he read Greats.

[5] In a long civil service career, Armstrong worked in several departments, including HM Treasury and the Home Office.

[10] In 1986, Armstrong was the key witness for the British Government as it sought to suppress the publication of Spycatcher, in which it alleged its author, Peter Wright, had attempted to disclose confidential information.

Wright's lawyer, Malcolm Turnbull, who later became the Prime Minister of Australia, was ultimately successful in lifting the publication ban.

[12] He was created a life peer as Baron Armstrong of Ilminster, of Ashill in the County of Somerset, on 26 February 1988,[13] and sat as a crossbencher.