He made 756 league and cup appearances in a 25-year playing career, scoring 27 goals. A defender, he began his career at Chelsea, turning professional in May 1990 and then making his debut in the Football League in April 1991. He played on loan at West Bromwich Albion from December 1991 and March 1992. He established himself in the first-team back at Chelsea during the 1992–93 Premier League campaign and went on to be named as the club's Player of the Year for 1993. He played on the losing side in the 1994 FA Cup final, before picking up a winners medal after Chelsea beat Middlesbrough 2–0 in the 1997 final. Chelsea also won the League Cup by beating Middlesbrough 2–0 in the final the following year, and Sinclair scored the opening goal in extra-time. Chelsea also won the UEFA Cup Winners' Cup in 1998, though Sinclair was ruled out of the final due to injury.