Cox was originally signed by the Detroit Lions as an undrafted rookie free agent on April 28, 2005, and he spent the 2005 training camp and preseason with the team. However, Cox was released on August 30, and he subsequently played in the CFL for the Montreal Alouettes. On January 9, 2007, he was signed by the Washington Redskins but was cut before training camp and returned to Montreal. After switching to the linebacker position for the 2009 season, Cox led the CFL in forced fumbles and capped off the year by winning the 97th Grey Cup with the Alouettes. In 2010, the Alouettes repeated as champions and won the 98th Grey Cup.