Johnson came through the youth ranks at his first professional club, Southend United. Johnson made his professional debut when he started the Football League Trophy match for Southend at home to Cheltenham Townin 1999. Southend lost 1–0 to go out of the cup, and Johnson played no more part in the 1999–2000 season. At the end of September in the 2000–01 season, Johnson came on as a substitute in the 1–0 win at Rochdale. By the end of January, Johnson had established himself as a decent defender and he played in a total of 26 games and scored his first goal in a 1–1 draw with Chesterfield at the Recreation Ground. In the 2001–02 season, Johnson made 34 appearances for Southend in all competitions. However, as Southend only finished 12th in League One, cost-cutting meant that Johnson was released from the Roots Hall club on a free transfer. He spent time on trial at nearby club Gillingham during the close-season summer of 2002. Johnson impressed in several friendlies for the Gills who soon signed him up on a full-time contract. Johnson was not seen as first choice for the Gills at the beginning of his career. His first competitive appearance in a Gills shirt came in the middle of September as a late substitute for David Perpetuini. Despite making 20 appearances that season as Gillingham finished a record high of 11th in the league. He never established himself as a first team regular, however and he only made 21 appearances the next season and only eight the season after that. It was, however, the 2005–06 season that saw Johnson stake a claim for a regular first team spot. Johnson made around 30 appearances in total, and perhaps would have made more were it not for three suspensions. Johnson scored his first goal for Gillingham against Milton Keynes Dons on New Year's Eve 2005 in a 3–0 win at Priestfield when he headed home Andrew Crofts' flick on following Michael Flynn's long ball.
Wycombe Wanderers
He was released by Gillingham in May 2007 and signed for Wycombe the following month. Johnson was a regular in the Wycombe team throughout the season and was awarded at the end of the year with the Fans' Player of the Year Award. He went on to sign a one-year extension, which kept him at the club until the summer of 2010. He then once again extended his contract for a further two years. He has been involved in two promotions at the club, having three managers in his spell at the club. In May 2014 Johnson was released after a series of injuries following Wycombe's final day league survival. When he left he had the second highest league appearance and fourth highest appearance total of any player at the club.
International career
Although he was born in England, Johnson qualifies to play for Grenada through his father. On 20 May 2011, he was included in the Grenada squad for the 2011 Gold Cup. Johnson played in all three of Grenada's games in the group stage, but the team was eliminated from the tournament after three heavy defeats.
