He began his career with Portadown in his native Northern Ireland from whom he signed as a trainee for Blackburn Rovers and spent seven years with the club, as well as spending time on loan with Nottingham Forest.

He played for his country at schoolboy and youth levels before representing the Northern Ireland senior team over an eleven-year period, earning 56 caps.

[1] His first English team was Blackburn Rovers, for whom Johnson signed as a trainee, from Portadown in 1997, making his debut in the League Cup on 30 September 1997, a 1–0 defeat to Preston North End, in which he picked up a yellow card.

Loaned to Nottingham Forest on 28 January 1998, Johnson played in five games, and, despite accruing a yellow card in his last February appearance, performed well for the team.

Despite not being part of the squad for the final Johnson had contributed to a few games earlier in the cup run, including scoring a goal against Manchester City.

[2] Having dropped down the pecking order at Blackburn and out of contract at the end of the season, Johnson signed for Championship club Birmingham City on 8 March 2002.

[13] Birmingham manager Alex McLeish described him as "always a terrier of a player [who] gave us fantastic commitment ... and played a significant part in us returning to the Premier League", and wished him all the best.

[29] In February 2024, he and fellow coach David Lowe took temporary charge of Blackburn for their 3–1 Championship win over Stoke City while waiting for new manager John Eustace to take over formally.