Defender Mills was stripped of the captaincy for his role in the infamous brawl at Bradford in March which also saw him hit with a six-match ban.
Mills was back from suspension for the final two games of the season but did not make another appearance for the club he joined from Rotherham in 2010.
Fellow defenders Dean Howell, John Dempster and Glenn Wilson have also been released at the end of their contracts along with midfielders Ben Smith and Byron Napper.
Staying on after the club took up the option to extend their contracts are Kyle MacFadzean, Claude Davis, Hope Akpan, Sergio Torres and Scott Neilson, whose goal against Accrington on Saturday secured promotion. Goalkeepers Scott Shearer and Michel Kuipers have been offered new deals.
Caretaker manager Craig Brewster said: "We have a good core of players to take us into League One next season and the task now is to add to the squad."