Two first-half Portsmouth goals were enough to hand the Cobblers their first defeat of the season, at Fratton Park.
Jed Wallace and Craig Westcarr scored to give Pompey a fully-deserved victory and one that will underline the feeling on the south coast that the club can mount a genuine promotion challenge this season.
For Northampton it was a night to forget, with home goalkeeper Paul Jones seeing virtually nothing of the ball and far too many of their players producing sub-standard performances.
It represents both their first loss of the campaign, the first time they have failed to score this term and only a second away loss with Town for manager Chris Wilder.
That it comes against an in-form and well-backed side with a squad that runs deep with quality will come as no consolation to Wilder, whose pre-match preparation was disrupted by the loss of midfielder Ian Morris to what will be a long-term lay-off with a dislocated knee.
The home fans were in good voice from the first minute of the game but it was the Cobblers who had the first good effort, Ivan Toney driving wide with a first-time shot when he had more time than he realised.
Ricky Ravenhill found space for a header at the end of a well-timed run but perhaps should have done better after making excellent contact on Evan Horwood’s swirling cross.
Pompey striker Westcarr had a shot blocked as the home side put together a couple of good counter-attacks and eventually got their rewards, with Nicky Shorey robbing Danny Alfei at right back and cutting a simple pass back to Wallace who drilled a shot across Matt Duke and into the far corner.
Again there will be question marks over Alfei’s defending - on this occasion he felt the ball would go out but Shorey’s experience told him otherwise and he nicked it away from him to leave the defender stranded on the touchline.
Then it was the turn of the previously in-form Kaid Mohamed to make a mistake, standing still to wait for a pass to reach him which allowed Ricky Holmes to cut it off and burst forward before drawing a strong save from Duke.
Collins then had to make a sliding clearance in his own six-yard box after the Town run had once again been sliced open, with Portsmouth knocking the ball around with a real swagger.
But it only delayed a second goal that felt inevitable, former Kettering striker Westcarr timing his run perfectly to beat the visitors’ offside trap and power a shot past Duke.
Lawson D’Ath was thrown on at half-time in place of Horwood and what was almost a diamond 4-4-2 system put into place.
Signs of encouragement were few and far between but there was evidence of a growing understanding between Toney and Marc Richards, with the two combining to put Mohamed in only for the winger to blaze a bouncing ball horribly off target.
Toney himself had a shot deflected wide midway through the second half, and although the corner came to nothing the 18-year-old deserves credit for fighting throughout the contest.
They never looked like getting into the game though, looking a bit short of ideas when getting the ball in attacking areas and never forcing goalkeeper Jones into meaningful action.
The visitors were fortunate the margin was only a two-goal one when Nigel Atangana curled a shot at the post when it looked easier to score and substitute Miles Storey almost headed past Duke after an uncertain moment in the away defence.
And so, the long wait for a Northampton win at Fratton Park goes on.
The Cobblers haven’t won on this ground since 1924 and that wait should be extended for some time, with Pompey on this occasion showing all the signs of a team with promotion capabilities.
Portsmouth: Jones; Devera, Robinson, Chorley; Devera, Dunne, Atangana, Wallace, Shorey; Taylor, Westcarr
Subs: Hollands (for Holmes 71), Storey (for Westcarr 77), Butler (for Taylor 90+1)
Not used: Brezovan, Whatmough, Barcham, Awford,
Cobblers: Duke; Alfei, Collins, Diamond, Robertson; Mohamed, Ravenhill, O’Toole, Horwood; Richards, Toney
Subs: D’Ath (for Horwood 46), Hackett (for Mohamed 64), Moyo (for Alfei 85)
Not used: Archer, Sinclair, Tozer, Nicholls
Cobblers fans: 366