Mathis, 32, played 10 seasons with the Jaguars after he was selected by the team in the second round of the 2003 NFL Draft. He became a free agent after the 2012 season though, as new Jaguars general manager Dave Caldwell allowed the veteran's contract to expire.
A Pro Bowler and First-Team AP All-Pro during the 2006 NFL season, Mathis has started in 129 of 137 career games and tallied 30 career interceptions, a Jaguars franchise record.
In the first two games of the preseason, the Lions defense allowed a combined 559 passing yards with five touchdowns and an interception. The Lions finished the 2012 season with the sixth-most points allowed.