In a game desperately poor on quality, the Cobblers were well-organised and fully worth their point.
Pompey's start showed promise as Ricky Holmes' run tricked him past two defenders to the byline where his cross was picked off as he lacked support.
Jed Wallace gave further cause for hope with a speculative effort from 25 yards out which caught keeper Matt Duke by surprise and bounced on to the top of the crossbar.
Northampton offered little going forward and their most dangerous threat came bizarrely from a long throw by Ben Tozer which bounced in the Pompey six-yard box and then on to the top of the bar with nobody applying a touch.
Portsmouth's increasingly ragged attacking efforts were summed up by a mishit Holmes corner which fell kindly for Sonny Bradley 20 yards out, only for his shot to strike team-mate Patrick Agyemang and bounce harmlessly wide.
It was an hour before either keeper was extended in to a save when Pompey keeper Trevor Carson clawed away a header from Cobblers' sub David Moyo and, encouraged by that, Matty Harriott then stung Carson's hands with a 25-yard shot as the visitors sought a winner.
Duke was forced into his first serious action on 64 minutes, diving full-length to clutch Therry Racon's raking 25-yard effort.
Northampton were left fuming when referee Gavin Ward waved away appeals as Pompey defender Sonny Bradley hauled down Moyo with a last-ditch challenge which stopped the striker as he bore down on goal.