Cousins graduated to the Brentford professional side in July 1989, making 21 Third Division appearances over the next two seasons.
In July 1991 he dropped into the Conference National to play for Martin O'Neill's Wycombe Wanderers. The club finished runners-up in his debut season, before they won the Conference and the FA Trophy in 1992–93. Back in the Football League, Cousins helped Wycombe to promotion to the Second Division in 1994 with their play-off final victory. This came despite his being stripped of the captaincy for a highly dangerous tackle on David Moss.[3] He was voted the club's Player of the Season in 1994–95 and 1998–99.[2] Remaining in the third tier for several years, he was a part of the club's remarkable FA Cup semi-final run in 2000–01, and played in the 2–1 defeat to Liverpool at Villa Park. In October 2002 he was given a testimonial match against Celtic,[4] then managed by his former boss Martin O'Neill.[5] Cousins made over 400 appearances for the club.[6]