The 23-year-old hit 13 Conference goals before joining Posh on loan from Grays earlier in the week and played his part in a decisive second half spell when Posh netted three times in four minutes.
Ironically, Accrington could easily have gone in at the break with a four-goal advantage but had to settle for one, planted firmly into the bottom of the Posh net by Ian Craney on 23 minutes. Leam Richardson lofted the ball forward and Craney had too much pace for Guy Branston and gave goalkeeper Mark Tyler no chance claiming his fifth of the season.
Craney should have doubled the Accrington advantage in first half added time but stabbed a left-footed shot wide of an upright after taking a Paul Mullin pass and rounding Tyler. In between Andy Todd rattled the home crossbar with a rising drive and Sean Doherty sent a 25-yard effort against the far post.
Peterborough created plenty of openings, Branston heading wide from a Jamie Day corner with Ian Dunbavin going full stretch to save from Richard Butcher.
Accrington had to reorganise at the back after losing central defender Robbie Williams with an injury after 34 minutes, but Phil Edwards slotted in effectively alongside Michael Welch who produced a superb stop to deny Richard Butcher in first half added time.
Trevor Benjamin blasted his ninth goal of the season to start the Posh revival with a 56th minute penalty, McLean scored from a tight angle after holding off Edwards a minute later and, on the hour, substitute Danny Crow tapped into an empty net after Dunbavin saved from McLean.
Craney put Accrington back in touch curling the ball directly into the net from a 79th minute corner, but Crow ensured Posh had the last word with a storming shot two minutes from the end.