The big defender headed an 88th minute Darren Ferguson corner past home goalkeeper Mark Tyler to ensure honours ended even.
Holt's effort denied in-form Peterborough a third straight League Two victory, but Mark Wright's men only have themselves to blame for failing to take maximum points which would have catapulted them into the play-off places.
Veteran winger Dave Farrell jumped to the top of the Posh scoring charts when slotting them ahead on the half-hour.
Impressive on-loan strike Lee Thorpe delivered an inviting low cross which Farrell slotted past keeper Michael Ingham for his fifth goal of the season.
Boss Wright was perplexed as to how Posh failed to double their lead just before the break.
Thorpe picked out Danny Crow with another pinpoint delivery from the right wing, only to see his strike partner thunder his shot against the crossbar from point-blank range.
The ball rebounded back into play but, while the Wrexham rearguard stood motionless, Posh midfielder Jamie Hand could only prod the ball into the arms of the falling Ingham.
Crow was again denied by the crossbar six minutes from time after being released by a defence-splitting pass from captain Dean Holden.
And the former Norwich City teenager was guilty of ignoring the perfectly-placed Sean St Ledger and instead dragging a shot wide in another lightning fast Posh attack.
Wrexham were just as wasteful in front of goal as two golden openings went begging in the second period.
Lee McEvilly intercepted a weak back-pass and danced round Tyler only to slice his shot into the sidenetting with the goal gaping at his mercy and fellow frontman Jonathan Walters shot wildly over from eight yards after being left in acres of space.
And Farrell's goal looked like being enough to earn another Posh win until Holt's late rescue act.