The giant defender was in the thick of a goalmouth scramble when he got the final touch and jabbed the ball over the line with a dozen players milling around him in the six-yard box.
Calum Willock had given Posh the lead midway through the second half with a first-time volley from Andy Legg's pinpoint corner.
Peterborough would have recorded a comfortable win if home keeper Matt Baker had not been in super form.
The Red Dragons second-choice shot-stopper had a busy night, starting with a superb save from Andre Boucaud to tip a 30-yard drive over the bar.
He scooped up a weak shot from Peter Kennedy and then tipped another angled volley from Willock away at full stretch.
He denied Boucaud again, held another shot from Willock and saw Clive Platt head narrowly wide before Willock finally opened the scoring.
With six senior players out injured, Wrexham had several youngsters in the side and came under heavy pressure later in the game.
They made few openings and looked no threat until manage Denis Smith brought on three subs in a late reshuffle.
Andy Holt had a header saved and Hector Sam missed a cross from two yards out in front of goal.