Priestley, known as Elvis to the Farrar Road faithful, only returned to the Welsh Premier League club in January after leaving last May to play for Prescot Cables and Stalybridge Celtic.
He also spent three seasons with Rochdale and also had loan spells at Scarborough and Chester City in 2000/1 before first joining the Citizens in August 2001 and was selected for the Welsh Premier League team of the season at the end of the campaign.
City boss Peter Davenport chose to replace regular 'keeper Richard Acton with Priestley mid-way through the last campaign.
FC United will play in the North West Counties League next season, sharing the ground of Droylsden