Clark was traded to the Washington Capitals as a restricted free agent on August 4, 2005, in exchange for a conditional pick in the 2006 Entry Draft. He scored 20 goals and 19 assists (39 points) with the Capitals in his first season, playing alongside rookie Alexander Ovechkin. The Capitals named Clark their new team captain on September 13, 2006. Clark then set career-high numbers in goals (30), assists (24) and points (54) during the 2006–07 season, continuing to play alongside Ovechkin.