Born in Carrickfergus, Co. Antrim in 1924, his family moved to Belfast where Armstrong attended Queens University. After two years he departed and spent 6 months at Belfast College of Art evening classes, where he met Gerard Dillon, also becoming acquainted with George Campbell and Daniel O'Neill. In 1946, he became a professional artist, but only surviving with clerical jobs, and in 1950 an exhibition of his work was held at the Grafton Gallery in Dublin. His work was also exhibited in Belfast in a Group Show in 1951 and in a solo exhibition in 1956. Armstrong first exhibited at the Royal Hibernian Academy in 1962 and up until 1995, contributed a total of 74 works at the RHA, becoming a member in 1972. He also taught painting at the National College of Art and Design in Dublin. In 1981, a retrospective exhibition was organised by the Arts Council of Northern Ireland, covering the years 1950 - 80, and in 1988 a studio sale of his work was held in Dublin.