Between 1949 and 1953, he was principal private secretary to the three successive Chancellors of the Exchequer Sir Stafford Cripps, Hugh Gaitskell and then R. A. Butler.

[1] During its dispute with the miners over the government's imposition of a Three-Day Week, however, Armstrong suffered a nervous breakdown.

[2] He returned to office after a period of sick leave but shortly after made known to his Second Permanent Secretary, Ian Bancroft, that he had been approached to accept appointment as Chairman of the Midland Bank.

There was internal discussion between Bancroft, the Cabinet Secretary, and the Prime Minister (by this time not Heath but Harold Wilson) as to the propriety of an official who was so close to the government's handling of economic affairs moving to the chairmanship of a clearing bank.

On 29 January 1975, he was created a life peer with the title Baron Armstrong of Sanderstead, of the City of Westminster.