The Cobblers twice hit the woodwork but failed to wipe out a precious Charlie Lee goal for their promotion chasing hosts.
Democratic Republic of Congo international Zakuani saw red from referee Clive Oliver when he clipped the heels of Ikechi Anya as the 21-year-old, making his first start, headed for goal.
The Cobblers almost made Posh pay as Danny Jackman beat Joe Lewis with the resulting free-kick, only to see the ball smack against the right-hand post.
Almost immediately an upright came to the rescue at the other end after Posh livewire Aaron McLean whipped in a left-footed shot that left goalkeeper Chris Dunn as little more than a spectator.
With Town making good use of their extra man Abdul Osman's cross found a way through the re-organised Posh defence, only for Anya and Colin Larkin to narrowly fail to make contact.
Posh grabbed the lead on 33 minutes with defender Lee taking time out from a superhuman effort at the back to go forward to claim his fourth goal of the season, nodding in a swirling Paul Coutts free-kick at the far post.
Cobblers altered their formation after the break and within a minute were denied by the woodwork yet again, Andy Holt heading a Jackman corner against the bar.
They wasted an even better chance on 70 minutes with substitute Leon Constantine failing to punish his former club when blazing a cross from Anya high over the bar.
Then in added time Constantine saw substitute Chris Westwood clear his shot from the goal-line.