Johnson was born in Kurri Kurri, New South Wales and played for Weston Workers Bears before moving to England to become a professional. He joined the youth ranks of Watford, and made his league début in the closing stages of the 1991–92 season. Johnson's ability to shoot accurately and powerfully from long distance secured him the "Goal of the Season" award for the 1994–95 season, for a 30-yard shot in a game against Wolves. Often in and out of the side for much of his early Watford career, Johnson became an integral part of Graham Taylor's re-shaped Watford side as it started the 1997–98 season. He formed a central midfield partnership with Micah Hyde that was to help drive Watford to the 1997–98 Division TwoChampionship. Johnson was also to show his long-range shooting skills in the 4–0 victory over rivals Luton Town at Kenilworth Road, scoring the first goal. The Hyde-Johnson axis continued into the 1998–99 season, with Johnson playing in Watford's drive to the play-offs and picking up a winners medal in the 2–0 final victory over Bolton. Johnson was to play a limited part in Watford's first Premiership season, with injuries sustained in August and January keeping him out for two months each time. Playing in a match against Manchester United on 29 April 2000 Johnson ruptured knee ligaments. He did not return until 28 April 2001, featuring in the final three games of the 2000–01 season. However, Johnson suffered a relapse, and he missed all of the 2001–02 season. He returned intermittently in the autumn of 2002, but after picking up another injury was sent on loan to Northampton Town for a month in February 2003 to gain fitness. He scored 1 goal - against Watford's rivals Luton Town - in 6 games for the Cobblers. Returning to Watford, Johnson featured in the final few games of the season, but still struggling for fitness and a first-team place he was released in October 2003. He had played 277 times for the Hornets in 11 years at the club, scoring 22 goals. He subsequently joined Colchester, playing a singular game for the Us in the Football League Trophy. After just a month in Essex, Johnson joined Stoke City. He played nine times for the Potters before joining Queens Park Rangers in February 2004, who were driving towards promotion from Division Two. Johnson played 11 times in Rangers' run-in, helping them to secure second position and automatic promotion. Johnson started the 2004–05 season in the first-team, but found the step-up to the Championship too demanding and slipped out of the team at the end of August. In October 2004 he was loaned to Milton Keynes Dons for whom he played three times. Returning to QPR, he was unable to regain his place and he was released at the end of the season having not played another game for the London club.
Johnson moved back to Australia, joining home-town club Newcastle United Jets in the newly formed A-League. Johnson played a key part in Jets' first season where the team managed a respectable 4th place under then manager Richard Money, starting 20 out of 23 games. However, in the 2006 off-season he moved to A-League club New Zealand Knights for the final year of their existence. In May 2007 Johnson signed for newly formed A-League team Wellington Phoenix FC. But he was then convicted of drunk driving in Newcastle and sentenced to 8 months imprisonment. He appealed and was subsequently given a suspended sentence and fine. He re-joined the Wellington Phoenix ahead of their debut A-League season in 2008. On 11 February 2009, Johnson announced he would be retiring from professional football at the end of the 2008–09 A-league season.
Non-playing career
On 28th January 2019, Johnson was appointed Technical Director at Wealdstone
