Posh led just before the hour when the defence failed to clear a Paul Carden corner as the ball bounced around a crowded goalmouth before Danny Crow tapped it home.
Wrexham's leveller came 10 minutes later when Mark Jones let fly with an angled shot that beat keeper Mark Tyler, hit a post, bounced out against the unfortunate Sagi Burton and rebounded off his chest for the own goal.
Jones had an early shot saved by Tyler and Wrexham were out of luck when Danny Williams turned a Darren Ferguson pass across goal only fro Mike Williams to miss contact at the back post.
In anither Wrexham raid they seemed certain to score when Lee Roche beat the keeper with an angled pass but Chris Plummer cut it out to deny Dean Bennett when he seemed certain to score.
Bennett had another shot blocked by the keeper after Jones put him through again.
Posh were relying on breakaways with Jamie Hand firing over from long range but Dean Holden was out of luck with a 20 yard drive which Michael Ingham saved at full stretch. Again when Crow rounded Danny Williams, Ingham was out quickly to save.
Peterborough improved after the break and Dave Bayliss had to be sharp to block shots from Carden and Adam Newton, but on the second occasion the Wrexham defender headed narrowly over his own bar as Holden closed in.
The hard working Lee McEvilly was always dangerous for Wrexham and brought a great save from Tyler, tipping a 15 yard shot over the top. The Posh keeper made another good save from Jones, when the Wrexham man turned and tried his luck from 20 yards.
Wrexham went all out for the winner in the closing moments swapping Robbie Foy for Dean Bennett and often looked in danger of over exposure in defence. In the last move of the game Matt Crowell denied Posh, turning Thorpe's shot off the line.