The deadlock was broken on 56 minutes when Michele Fornasier's stray pass out of defence put the Reds on the back foot and Boyd lashed a deflected shot from distance past Johnstone. Another wayward ball from the centre-back position, by Thorpe, almost gifted a second to Tomlin but he dragged his effort wide.
United tried to mount a response and Macheda found the impressive Cole on the right before hammering the wide man's cross goalwards, only for Day to dive to his left to pull off a quite magnificent save. Joshua King's introduction, at the expense of left-back Tyler Blackett, added pace to the attack but Posh were rarely troubled again until the closing few minutes.
Reds Academy product Ajose wrapped up the victory on 82 minutes by calmly beating Johnstone after being set up by Paul Taylor's astute pass to promote his claims to stay at London Road. Taylor forced Johnstone into more action before the final whistle as Darren Ferguson's side deserved the triumph in the end, while King failed to provide the finishing touch after a couple of positive runs at the death.
United: Johnstone; Vermijl (Veseli 73), Fornasier, Thorpe, Blackett (King 66); Cole, Tunnicliffe, Petrucci, Powell (Agnaldo 79), Lingard; Macheda. Subs not used: Sutherland, Gladstony, James.