It was a frustrating afternoon for Darren Ferguson's men as they huffed and puffed through 87 minutes of this dour Bank Holiday battle without threatening a breakthrough.
But all that changed as hot-shot striker Craig Mackail-Smith and veteran defender Alan White were responsible for two goals in the final three minutes at London Road which made sure Posh won for the first time in three matches.
Mackail-Smith broke the deadlock when benefiting from a goalkeeping howler from Mansfield number one Jason White, who allowed the former Dagenham man's low shot to squirm under his body at the near post.
The Stags stopper partly made amends with a smashing save to thwart Posh skipper Adam Newton seconds later, but he was powerless to stop his namesake Alan clinching the points in the 89th minute.
Alan White rose above the Mansfield defence to powerfully head home a cross from substitute Jamie Day and make sure the frowns turned to smiles on the faces of the home fans.
Posh had earlier squandered a series of good openings in the second half as Mansfield bravely battled to hang on to a point.
Stags defender Alex John-Baptiste bravely blocked a Newton drive before substitute Danny Crow failed to pull the trigger in time.
Newton wildly volleyed over from close range in the 65th minute and, when Posh finally did manage an effort on target, keeper White charged out to smother a Boyd shot soon after.
For Mansfield it was a third match in a row without finding the net and they scarcely looked like testing a strong Posh rearguard.
Big striker Barry Conlon looped a first half effort over the bar before substitute Nathan Arnold dragged a decent chance wide in the second period.