Armstrong started his career as a junior for Brighton & Hove Albion youth team, progressing into the first team in 1997. In March 1998, he was picked for the Republic of Ireland under-21 team for a friendly game against the Czech Republic. Armstrong scored his first senior goal from a penalty in a game against Chester City in January 1999. Armstrong made over 50 league appearances with Brighton and Hove until he was released by Micky Adams in 2000.
