The six-foot-five, 316-pound Mitchell was a CFL all-star in 2011 after registering six sacks and 33 tackles in helping B.C. win the Grey Cup. But last season he was suspended for two games by the league after hyperextending the arm of Edmonton Eskimos offensive lineman Simeon Rottier during a game.
He was also fined by the CFL last October for tweeting a term that was derogatory toward Chinese. Mitchell later apologized, saying he didn't know the word was offensive.
The 27-year-old from Virginia Beach, Va., was due to return from injury that week, but the Lions kept him off their active roster following the league's decision to fine him.
Mitchell has registered 54 tackles and eight sacks in 34 career CFL games. Prior to joining the Lions, Mitchell spent two seasons on the practice roster of the NFLs San Francisco 49ers.
"Khalif has shown the ability to be a dominant interior defensive lineman in the CFL," Argos GM Jim Barker said in a statement. "He has been a part of a Grey Cup championship team and understands the level of commitment needed to again be a champion.
"We're excited to have him join our club."
The move certainly fills a need for the defending Grey Cup champions. In the off-season, veteran defensive tackle Adriano Belli retired, defensive end Ron Flemons was released and impressive rookie defensive tackle Armond Armstead signed with the NFL's New England Patriots.
It won't take Mitchell long to face his former team. The Lions' home opener will be July 4 against Toronto.
B.C. has retooled its defence this off-season, having previously released veteran cornerback Byron Parker and linebacker Anthony Reddick.
But the club's defensive line still features defensive ends Keron Williams and Khreem Smith along with young tackle Jabar Westerman of Brampton, Ont., who B.C. selected second overall in last year's CFL draft. B.C. also signed defensive end Julius Williams as a free agent in February after he spent two seasons with the Edmonton Eskimos.