Cheshire-born Turnbull is a product of Stockport County's youth system and became the youngest player to represent the Hatters in the Football League, making his first class bow as a 16-year-old in 2005.
Gaining experience on loan at Altrincham in the 2007/08 season, Turnbull went on to make over 150 appearances for Stockport before joining Northampton in the summer of 2011.
He played 14 times in League 2 for the Cobblers last season but finished the campaign chalking up a similar amount of appearances back with Stockport, who he joined on loan during the January transfer window.
Turnbull returned to Edgeley Park for a third spell in September and was on the scoresheet in County's 3-3 draw at Sincil Bank a month later before returning to Northampton following the completion of his 93-day loan.
He has been handed the number 35 shirt and will go straight into the squad for Saturday's visit of Forest Green Rovers.