The decision to end the midfielder's time in Paisley despite having a further year still to run on his contract had been expected, since the 29-year-old featured only intermittently under Danny Lennon last season before ending the latter part of the campaign on loan at Oldham Athletic.
The Sky Bet League 1 club are expected to make an approach to sign Harkins on a permanent deal after he impressed during that loan spell, while the club also completed the signing of former Kilmarnock midfielder and Scotland Under-21 internationalist Liam Kelly.
Two other former Kilmarnock players have also settled their futures. Colin Nish has signed a new one-year contract with Dumbarton and the 33-year-old striker will continue to combine his responsibilities as a player with a role coaching the club's under-20s.
His erstwhile Kilmarnock team-mate James Fowler will also spend next season in the SPFL Championship after joining Queen of the South as player-coach. In League 2, meanwhile, new Arbroath manager Allan Moore has appointed Todd Lumsden as his assistant.