Armstrong played as a fullback for his first club Western Suburbs, where he made two first-grade appearances. He was one of the club's best players in their 1981 reserves premiership team, scoring a try and kicking five goals from five attempts in the grand final win over Parramatta, which saw him signed up by Canterbury soon after.[2][3]
Over two seasons at Canterbury, Armstrong made a total of 33 first-grade appearances as a goal-kicking winger, three of which came in the 1983 finals series.
Armstrong joined Cronulla in 1984 and finished the home and away season as the competition's leading point-scorer. He amassed 186 points in 23 games, to edge out former Canterbury teammate Steve Gearin for the accolade.[4]
In 1986 he was signed by Illawarra and played six first-grade games before suffering a knee injury in round seven.[5]