They saw their slender lead disappear when Craig Mackail-Smith grabbed a last-gasp equaliser to deny them.
Then in the dying seconds, Darlington substitute Clark Keltie saw his free-kick deflected onto the post.
But Darlington deserved no more than a draw as Peterborough dominated from the off and it was totally against the run of play when Tommy Wright headed home Julian Joachim's pin-point cross from the right on 64 minutes to give them the lead.
And they would probably have held on to win had substitute Xavier Barrau not been sent off on 75 minutes.
He had already been booked when he needlessly clattered into Peterborough defender Chris Westwood on the halfway line.
And Peterborough made the extra man count when Darlington failed to clear in a goalmouth scramble and Mackail-Smith popped up to crash a close-range shot into the net.
Peterborough could have scored just after the start of the game when Darlington keeper Andy Oakes was called on several times to make excellent saves.
Starting just his second game of the season, he was in the thick of the action inside three minutes when he dived full-length to keep out Charlie Lee's rasping low drive which was heading inside the post.
Before Oakes could draw breath he had to block another Lee shot.
Then the keeper did even better on ten minutes after somehow keeping out a close-range volley from Dean Keates who had been set-up by Jamie Day.
In the first half Darlington were restricted to long-range efforts from full-back Tim Ryan.
Darlington were well short of their best and clearly missed top-scorer Pawel Abbott, who twisted his knee in the pre-match warm-up.
Peterborough also lost a player before the game had even started as keeper Mark Tyler suffered a back injury so Shwan Jalal took his place in goal.
However, the stand-in keeper had very little to do in a game his side dominated.
Apart from taking one cross, his first piece of action was picking Wright's header out of the net, before Peterborough finally got their reward for a good display.