London-born Johnson is the club’s second longest-serving player, having joined from Gillingham in 2007.
The centre-back collected the Supporters’ Player of the Season at the end of his debut campaign under Paul Lambert, and continued to impress in Peter Taylor’s promotion-winning squad before repeating that achievement under Gary Waddock, featuring in all 10 games of the club’s unbeaten run at the end of the season.
Johnson made his international debut for Grenada in the CONCACAF Gold Cup in the summer of 2011, after having earned a new two-year deal to remain at Adams Park.
Made 21 appearances for the Chairboys last season, taking him to 170 games in all competitions for the club.