Born in England, Armstrong became a naturalised Finn in the early 1990s,[3] and enjoyed a long and successful coaching career in Finland with Rovaniemen Palloseura, TP-Seinäjoki, FC Haka and most recently HJK Helsinki.

[1] He began his football career as an apprentice with Sunderland, making his debut in the Second Division on 4 October 1977 as a substitute in a 1–1 draw at home to Cardiff City.

Subsequently, he played in nine different clubs around Finland, appearing in 179 league matches, scoring 60 goals, and winning three championships (with OPS in 1979 and 1980 and with Kuusysi in 1984).

In 1997, he was offered a post at FC Haka, which he accepted, getting the club promoted from the second tier and winning three league championships in a row with the Valkeakoski team.

[3] After working as sporting director of SJK for two years, on 24 October 2014 Armstrong was appointed as the new manager of Ilves Tampere for the upcoming season.