On 31 August 2007, Johnson signed for Portsmouth on a four-year contract for a fee believed to be £4 million. He was handed a start in the first match after his transfer, a 3–1 defeat at Arsenal, and became the club's regular right back. On 20 October, Johnson scored his first goal in over three years, against Wigan Athletic in a 2–0 win, after dribbling from the halfway line. He helped reach the 2008 FA Cup Final against Cardiff City where Johnson played in a 1–0 victory. Shortly after Johnson's cup success, he and Portsmouth substitute goalkeeper Jamie Ashdown had their winner's medals stolen from their hotel rooms. After the medals were not returned, the FA remade medals for Johnson and Ashdown.