He started his career at Bury as a trainee in August 1999, playing 33 games and scoring 1 goal before joining Oldham Athletic for £200,000 in October 2001, in only his second season as a first-team player.
He eventually returned to regular first team football after battling back bravely from career-threatening knee injury problems that limited him to just 13 games in his first season with the Blades and ruled him out for the whole of 2004–05.
After a brief spell at Blackpool to improve his match fitness earlier in the season,[4] Armstrong became a valuable member of the team that gained promotion back to the Premiership in 2005–06.
With his first team options limited at Bramall Lane he was allowed to leave and signed for Reading in August 2008 for an initial fee of £500,000 with the potential for it to rise to £800,000 depending on conditions.
[10] Armstrong received an international cap for Scotland B, after being included in the starting eleven in a 1–1 draw, against the Republic of Ireland B team, at the Excelsior Stadium on 20 November 2007.