The Town slipped to their sixth defeat on the spin as they surrendered the lead at Sixfields to drop out of the Play-Off places.
Alex Lawless curled the Hatters in front just 36 seconds into the second half but, the home side – themselves chasing a place in the top seven – levelled when Ricky Holmes converted a 64th-minute penalty after Michael Harriman was unfairly adjudged to have handled in the area.
And although the Town tried to get a winner, it was the Cobblers that took all three points when James Gray struck four minutes from time to condemn the Hatters to yet another defeat.
The Hatters began showing five changes to the side that lost at home to Wycombe on Tuesday with Aston Villa defender Lewis Kinsella given his debut at left-back. The Town also welcomed back midfielders Nathan Doyle and Jonathan Smith, while Mark Cullen and Lawless also started.
In what was John Still’s 100th league match in charge of the Hatters, it was the Cobblers that shadowed an even first half in which Elliot Justham was the busier of the two goalkeepers.
The Town stopper was called into action in the second minute to stop Joel Byrom’s daisycutter from the edge of the box.
Kinsella, in at left-back for the injured Jake Howells, was making his senior bow, and 11 minutes into it he was shown a yellow card for hauling down Holmes down the Cobblers’ right.
The Hatters’ defence was under pressure in the 12th minute when, from the free-kick in which Kinsella was carded, John-Joe O’Toole stabbed an effort across goal – but no teammate in maroon was on hand to turn the ball home.
Two minutes later the Town forced their first chance; Cullen escaping the home defence momentarily to drive a left-foot shot into the arms of home stopper Matt Duke.
Clear-cut chances, however, were at a real premium, especially for the Hatters, Cullen again guided a shot goalwards after seizing on Paul Benson’s knock-down but Duke was again equal to it.
As half-time approached the Hatters won a series of corners which put the Cobblers on the back foot. However, after one flag-kick was cleared, the hosts broke and Smith’s inadvertent sliced clearance found O’Toole unmarked inside the box – but thankfully for the Town the midfielder’s firm hit was held well by Justham.
Justham was then called into action twice more before the interval – once to gather Gray’s impudent flick following Jason Taylor’s left-wing raid and then diving on another low drive from Byrom from the edge of the penalty area.
However, the Town started the second half with a bang and took the lead just 36 seconds after the restart. Lawless received the ball on the left-hand edge of the penalty area and then curled a delicious effort into the top corner of the net with Duke beaten. It was the midfielder’s third goal of the season in just his fifth start.
Like he did in the first half, Justham was aware to the danger on 50 minutes to keep the Town’s lead intact as the hosts went in search of a quickfire leveller but the Hatters were dealt a blow three minutes before the hour when skipper Stephen McNulty picked up an injury and he was replaced by Alex Lacey.
But the Town’s lead was cancelled out on 64 minutes when the hosts were awarded a penalty when Harriman was adjudged to have handballed in the box and Holmes blasted the resultant spot-kick, giving Justham no chance.
With their tails up, the Cobblers went in search of the lead and the Town were indebted to a superb save from Justham on 68 minutes when he kept out Byrom’s close range effort after the hosts had worked a smart move from a free-kick.
The Town came close to sniffing out a second for themselves, only for the ball to drop out of reach when pinging around in the penalty area, and with 20 minutes left, Still sent on Ricky Miller for Cullen.
But the Hatters needed Justham to save once more from Byrom on 72 minutes as the Cobblers pressed and home sub Ivan Toney should have done better when well-placed six minutes later when he nodded wide with the goal at his mercy following a dangerous right-wing cross from Holmes.
As time ticked down the home side continued to push forward, and with four minutes to go the Cobblers took the lead. Justham kept out a heavily deflected shot from range, but Gray was first on the scene and after his first shot was blocked his follow-up was fired beyond Justham to make it 2-1.
Five minutes were added on at the end of the 90 but the Hatters couldn’t find a late leveller and they were left to reflect on a sixth successive defeat in League 2 to leave them out of the Play-Off places with seven games to go.
Town: Justham; Harriman, Kinsella, McNulty (sub Lacey 57), Wilkinson; Smith, Doyle, Drury, Lawless (sub McGeehan 82); Cullen (sub Miller 70), Benson.
Subs not used: Guttridge, Stockley, Hall, Gooch.
Attendance: 5,668, the highest of the season at Sixfield, helped by 913 supporting the Hatters.