Gwion Edwards’ superb goal from a free-kick was not enough to pick up points as Crawley Town suffered their fifth successive defeat.
After a goalless first half, Ricky Holmes gave Northampton the lead three minutes after the restart from the penalty spot.
The Cobblers made it 2-0 with ten minutes to go through a scrappy second goal by John-Joe O’Toole.
Crawley were reduced to ten men for the last four minutes when Andy Bond was sent-off.
Reds made two changes to the side with Shamir Fenelon and Frankie Sutherland returning to the starting XI in place of Lee Barnard and Lyle Della-Verde.
Skipper Sonny Bradley reverted to his regular defensive role after playing Saturday’s defeat at AFC Wimbledon up front.
Returning home for the first time since clinching the title, the Cobblers were given a heroes’ welcome by a buoyant Sixfields and their new status as champions was reflected in their dominant performance as Crawley were outplayed and outclassed throughout.
It could have been more than two with the woodwork rattled on more than one occasion before Crawley, who had barely threatened, gave themselves a sniff through Gwion Edwards’ late free-kick.
Bond saw red for a poor challenge straight from the restart and that ended any lingering hopes the visitors had of snatching what would have been the unlikeliest of draws.
It wasn’t their most polished of performances, but Northampton didn’t need to get out of second gear against a poor Crawley outfit.
Despite making hard work of it, the Cobblers were worthy winners as they ended a run of four straight draws with a victory that takes them up to 92 points and keeps their hopes of reaching 100 points alive.
Oozing all the confidence of a side who’ve just clinched the title, Cobblers made a purposeful start and applied all the early pressure with John Marquis side-footing a second-minute effort not far wide.
Whilst clear-cut chances were at a premium throughout the opening 45 minutes, Northampton were playing with an attacking zest and popped the ball around with ease as Crawley spent much of the first-half chasing shadows.
All they were lacking was killer final ball as O’Toole’s well-struck first-time shot from distance was well held by Jack Rose, before Marquis was twice crowded out when trying to get a shot away inside the penalty area.
At times, this game had a distinct end-of-season feel about it and rather laboured along but occasionally the Cobblers strung together some wonderful passages of play with Danny Rose, Ricky Holmes and Hoskins particularly combining well.
Marquis had another effort deflected behind for a corner following a slick move before Hoskins twice went close in as many minutes, first denied by Rose and then blazing over having found space inside the penalty area.
Crawley offered nothing in the way of an attacking threat, but for all Northampton’s possession and good play, it took the visitors just 90 seconds of the second half to test Adam Smith for the first time when the Cobblers keeper produced a fine low save from Edwards who had sneaked in behind the home defence.
He didn’t know it at the time, but the importance of that Smith save proved pivotal as only seconds later Northampton broke and won a penalty up the other end.
Former Crawley player Nicky Adams was wrestled to the ground inside the box, leaving the referee no choice but to point at the spot and Holmes confidently stepped up to dispatch the spot-kick right into the corner, giving Rose no chance.
Now ahead, Northampton continued to dominate both possession and territory and they were inches from a second when Crawley captain Sonny Bradley inadvertently deflected Holmes’ teasing cross just wide.
Holmes had tormented the visiting back-four all evening, constantly popping up in pockets of space and driving towards the Crawley goal, and while Crawley showed a fraction more ambition after falling behind.
Town remained in the ascendency and only the crossbar prevented them from adding a second when McDonald’s rasping drive from an acute angle flicked off O’Toole and rattled the woodwork.
The crossbar was shaking for a second time moments later when Holmes’ ambitious free-kick floated over everyone and clipped the top of the goal before bouncing away.
A second goal was inevitable, though, and it duly arrived 10 minutes from time when Crawley failed to deal with a corner and O’Toole scrambled home from close-range after McDonald’s initial shot had been blocked.
Crawley looked dead and buried but they gave themselves hope of snatching an unlikely draw when, with six minutes remaining, Edwards’ sweetly-struck free-kick found the bottom corner.
Their chances of a fightback were dealt a blow just seconds later, however, when a needlessly reckless lunge from Andy Bond on Alfie Potter resulted in a straight red card and that virtually ended any hopes of a comeback as Northampton comfortably held on.
Crawley: Rose, Oyebanjo (Young 87), McNerney Dunne, Bradley (c), Smith, Bond, Fenelon (Deacon 81), Sutherland (Ashton 81), Edwards, Barnard
Subs not used: Walton, Preston, Yorwerth, Della-Verde
Cobblers: Smith, Lelan, Diamond, McDonald, Buchanan (c), O’Toole, Rose, Holmes (Cresswell 90), Adams (Potter 66), Hoskins, Marquis (D’Ath 81)
Subs not used: Clarke, Taylor, Byrom, Collins
Referee: Iain Williamson
Crawley fans: 50
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