Andy Clarke's 15th goal of the season earned Posh a precious win over their derby neighbours in this tight Nationwide Division Two relegation battle.
Clarke cashed in on a mix-up between defender Paul Reid and goalkeeper Nathan Abbey ten minutes after the re-start and could afford the comfort of rolling the ball into the net.
The victory meant Posh leapfrogged above the Cobblers in the table while stretching their unbeaten away run to four matches.
Before dashing off to watch Luton down the M1, Posh boss Barry Fry said: "We made heavy weather of that but the lads battled hard and we deserved it.
"That puts the pressure on Northampton in the relegation fight." Cobblers manager Terry Fenwick has tasted defeat in his first two matches since succeeding Kevan Broadhurst in the Sixfields hotseat.
He believes his side need to show more composure if they are to hold a potentially disastrous slide.
The Cobblers have not scored at home since beating Port Vale on November 23, and this latest shut-out extended the agony for home fans to more than seven and a half hours.
Fenwick saw striker Derek Asamoah calve out a brilliant first-half chance, weaving in along the byline before rounding Lee Harrison, only to shoot into the side netting with the goal gaping.
That is typical of Northampton's luck at present, and Harrison proved a star turn in the second half to save from Darryn Stamp and Paul McGregor as well as making a crucial touch to take the ball from Asamoah's toe.
A miserable afternoon was made worse for Northampton when Stamp was given his marching orders by referee Paul Robinson with 21 minutes left.
Booked in the first half for a foul, Stamp was pulled up by Robinson after catching Posh defender Simon Rea.
Northampton have now tasted victory just once in 11 matches and face a tough schedule in the next month against Crewe, Cardiff, QPR and Luton.