Darren Ferguson collected his first win as Peterborough manager to leave his old club in the relegation mire.
Aaron McLean got Posh on their way, firing past debutant Wrexham goalkeeper John Ruddy after collecting an Adam Newton cross.
Lee McEvilly fluffed a good chance for the visitors, before George Boyd got his first Posh goal, beating Ruddy from a difficult angle.
Steve Evans put through his own net to compound Wrexham's misery near the end.
Peterborough: Tyler, Newton, Arber, Morgan, Blackett (Futcher 90), Low, Hyde, Butcher, Day, Mclean (Crow 79), Boyd (Mackail-Smith 80).
Subs Not Used: Strachan, Jalal.
Goals: Mclean 23, Boyd 53, Steve Evans 82 og.
Wrexham: Ruddy, Spender, Pejic (Done 43), Steve Evans, Mike Williams, Mark Jones (Marc Williams 73), Danny Williams, Garrett, Barron, Roberts (Johnson 58), McEvilly.
Subs Not Used: Ingham, Samba.
Booked: Mark Jones, Danny Williams, Garrett.
Ref: P Melin (Surrey).