Mills, still only 29, signed for the club for a second spell from Macclesfield but with his short term contract due to expire this week, he has been allowed to leave.
Boss Steve Evans continues his clear out of the squad with several loanees expected to move elsewhere on permanent deals this week as fresh faces arrive.
Kayode Odejayi, Matt Tubbs and David Worrall have all left the club in the past few days, with Daniel Nardiello and Lionel Ainsworth not expected to return from their loan stints at Bury and Motherwell.