 Armstrong was elected to the Christchurch City Council from 1919 to 1925, and again from 1927 to 1929.Whilst a member of the city council he was a perpetual advocate for public works schemes and municipal housing to both create employment and improve the quality of life for workers.In 1919 he became secretary of the Christchurch Tramway Employees' Union, holding the office until 1922 when he resigned.