Mitchell was signed by the BC Lions of the Canadian Football League on September 21, 2010. He was added to the team's 46-man playing roster as a nose tackle and activated to the team's 42-man game day roster against the Calgary Stampeders for September 25. With limited playing time in the 2010 CFL season Mitchell only recorded 6 tackles. Mitchell's second year in the league proved to be a breakout campaign as he earned 2011 CFL All-Star honors with 33 tackles and 6 sacks. Despite popular opinion which suggested that Mitchell would try his luck with an NFL team after an outstanding CFL season, he instead re-signed with the BC Lions on January 16, 2012.
Mitchell was suspended by the CFL on July 23, 2012 for two games without pay after the B.C. Lions defensive tackle violently hyperextended the arm of Edmonton Eskimos offensive lineman Simeon Rottier.
Caused a local controversy for what were interpreted as racist remarks he made on Twitter about the 2012 US Presidential Election debate between Mitt Romney and Barack Obama, resulting in a one game suspension and fined an 'undisclosed' amount of money. It started when he tweeted ""My Teamate said who I think Won the debate. I said the American Ppl. Cause if they watched carefully BOTH OF THEM Hide Money with the Chinks". He later explained "So I Never knew "Chink" was racist. I def. meant no harm. I was referring to Obama & Romney putting their personal trust in Chinese Banks.".