In 1934, he was elected as an alderman of Sydney Municipal Council, representing the Australian Labor Party until 1948. Armstrong was selected for Labor"s slate of candidates for the Australian Senate for the 1937 election partly because his name would appear high on the alphabetic ballot and he was duly elected, effective from July 1938. During 1945, Prime Minister John Curtin"s health greatly deteriorated but politicians and the media declined to publicly discuss Curtin"s health for fear of concerning the Australian public during World World War World War II As a result, Armstrong gave a speech in the Senate on 13 June which included the first public reference to Curtin"s health, and left the Australian public surprised.
He was appointed Minister for Munitions in Ben Chifley"s November 1946 ministry.
In April 1948, his portfolio was merged with the Supply functions of Bill Ashley"s portfolio to create the portfolio of Supply and Development and he was attacked by the opposition for the breadth of his powers. Following Labor"s defeat at the 1949 election, he became deputy-leader of the Opposition in the Senate.
He was relegated to an unwinnable fourth position on Labor"s ticket for the 1961 election and left parliament in July 1962. Armstrong was elected Lord Mayor of Sydney in 1966, but the position was abolished by the Askin government in 1967.
The Whitlam government appointed him Australian High Commissioner to the United Kingdom from 1973 to 1974.