As well as Mulligan, Joe Cardle, Robin Hulbert, George Pilkington, Adam Eckersley, Craig Rocastle and Colin Miles have all been told their futures lie away from Vale Park.
Mulligan, a New Zealand international, moved to Vale Park on a free transfer in January.
He haf been out of favour at Glanford Park since breaching club discipline ahead of a crucial clash with Bristol City last season.
Earlier this month Jason Talbot, Charlie O'Loughlin and Lloyd Ransome were told they were not being retained by Vale, while loanees Keith Lowe and Mitch Hanson have returned to their parent clubs.
Harsley, Danny Whitaker, David Howland and Luke Prosser have been offered new deals while Ross Davidson has already signed a new contract.