Ali Gerba put Dons ahead in first-half stoppage time before Jason Taylor equalised midway through the second half, but Jim Gannon’s men completed the comeback when Turnbull tapped home Tommy Rowe’s inviting cross.
County were given a scare early on as Luke Chadwick hit the bar before Anthony Pilkington shot straight at Willy Gueret when clean through on goal.
And Pilkington was left to rue his miss as the hosts took the lead minutes later when Peter Leven played a superb slide rule pass for Gerba to prod home.
Gannon sent his men out early from half-time as he looked for a response and he got it when Taylor tapped in at the back post after Pilkington’s corner was not cleared.
Kevin Gallen, Miguel Llera and Chadwick were then all guilty of missing superb chances late on and they were punished as the hapless Lewington deflected Rowe’s dangerous cross past his own goalkeeper.