After scoring one goal, having two disallowed, hitting the woodwork and missing a penalty, Chesterfield were surprisingly trailing at the break but Jack Lester was chief architect as they exposed home defensive limitations in the second half to claim a first league success of the new campaign.
Aaron Mclean fired Darren Ferguson's side in front on nine minutes, looping a header over goalkeeper Barry Roche from Adam Newton's right wing cross.
By the time Lester levelled from four yards ten minutes later, the former Nottingham Forest front man had seen one effort ruled out for a foul and missed a penalty.
The lively Lester failed to beat Posh's Shwan Jalal from the spot after he had been impeded by Chris Westwood and once on terms Chesterfield were denied when Aaron Downes ran in from the right to send in a thumping header that rebounded off the bar.
Posh edged in front again in the fifth minute of first-half added time with Charlie Lee claiming his first league goal with a shot from a dozen yards after a neat build-up involving George Boyd and Rene Howe.
Chesterfield then struck twice in three minutes to grab the lead, Jamie Lowry firing into the roof of the net after Jalal could only push away a 59th minute shot from Lester who showed great skill and control to create the opening.
Posh were given no time to recover with Derek Niven running in impressively to head in Gregor Robertson's centre for a lead they retained to the end.