Johnson was once one of the NFL's most punishing backs rushing for 1,789 yards and 17 touchdowns in 2006 and 1,750 yards and 20 touchdowns in 2005 with the Kansas City Chiefs.
But in the four seasons since Johnson has never again come close to displaying the same devastating form.
Last season, he appeared in two games for the Washington Redskins and carried the ball just five times for two yards.
Terms of the deal were not disclosed.
(Writing by Steve Keating in Toronto; Editing by Frank Pingue)