McGleish confidently rolled home his 22nd goal of the season as Wycombe denied Posh a win.
Stefan Oakes’ superb free-kick had put Wycombe in front early on but second-half goals from George Boyd and Liam Hatch had seen Posh take the lead.
Oakes hit his curling left-foot effort in from 25 yards after a foul on Leon Knight with just five minutes on the clock.
Boyd made the breakthrough for United on 66 minutes with a strike from the left side of the box that was deflected in by centre-back Mike Williamson.
Hatch’s goal in the 84th minute, when he headed in from Charlie Lee’s cross, looked to have won it for Peterborough, but McGleish stroked in the equaliser from the spot after a foul by Josh Low.