Cobblers' reunion with former boss Chis Wilder ended in late heartache as Kieron Freeman hammered home an 88th minute winner to clinch a 1-0 victory for Sheffield United who had toiled for much of Saturday's hard-fought tussle at Bramall Lane.
Just as they did at Oxford five days earlier, Northampton produced a dogged and determined performance which frustrated their title-chasing hosts for large parts.
The Blades monopolised possession throughout and launched wave after wave of attack but their threat was repelled by the Cobblers, who looked on course for a point when heading into the final few minutes.
There was to be a late sting in the tail, however, as Freeman pounced on a loose ball inside the penalty area and kept his composure to beat Adam Smith and ensure Wilder enjoyed a happy reunion with his old club.
It was harsh on Northampton who had restricted United to very few clear-cut chances throughout the 90 minutes despite the home side's intense pressure, however Freeman's goal meant it was all in vain.
Earlier, Rob Page made only one change to the team which beat Oxford United by recalling fit-again Sam Hoskins at the expense of Alfie Potter, who was unfortunate to drop to the bench.
Hoskins lined up on the right side of midfield in a 4-4-2 but Northampton were immediately forced on the back foot by United and had to withstand some early pressure as Sharp screwed well wide and then Paul Coutts drew Adam Smith into action for the first time.
The Cobblers replied with a couple of half chances of their own when Paul Anderson's fine run and cross culminated in Hoskins having a shot well blocked before John-Joe O'Toole almost latched onto Matty Taylor's corner.
Back came United through John Fleck, who at times ran the show from midfield with his impressive range of passing, and he set up an attack which almost saw the Blades strike as Coutts' side-footed attempt from 20 yards drifted a fraction wide, much to the relief of Smith who was rooted to the spot.
The lively Anderson was again in the thick of the action when heading Aaron Phillips' cross wide but that was largely against the run of play as the Blades continued to make most of the running.
Mark Duffy's free-kick only just missed Smith's right-hand post via a deflection in another example of the danger posed by United who were now firmly on top and pushing for the breakthrough.
But despite their stranglehold on possession, Wilder's side were being restricted to speculative pot shots with Duffy and then Sharp the next to try their luck without success.
Then followed a rare period of Cobblers pressure during which O'Toole again came close to meeting Taylor's corner before a terrific driving run from Hoskins nearly teed up Anderson at the back post.
The remaining 15 minutes of the first-half were more sedate as the game threatened to burst into life without ever really catching fire, much of that owing to Northampton's robustness in defence which blunted the home side's threat.
The second-half picked up from where the first left off and with United's attacks continuing to break down, the Bramall Lane natives grew increasingly restless with the lack of goalmouth action.
Despite the game now resembling an attack versus defence training drill, the hour-mark came and went without incident before United were within inches of finally making their dominance count.
A deep, floated cross to the back post was met by Sharp who headed against the woodwork before the follow-up effort was hacked away by some frantic last-ditch defending.
Attacks at the other end had dwindled to the point where they were almost non-existent but Northampton almost hit their hosts with the ultimate sucker-punch nine minutes from time when Taylor's superb hit from a 35-yard free-kick was brilliantly tipped over by Simon Moore.
That was the first piece of strenuous action for either goalkeeper and Moore's opposite number Smith was then on guard to watch Duffy's fierce drive whistle not far wide of his goal.
With the clock ticking down, it seemed Northampton would hold on for a hard-fought point but all of their hard work was undone just two minutes from time in a galling finale for the 1,650 away fans.
United's pressure was now relentless and finally they found a way through their stubborn visitors when Northampton failed to clear a cross and the ball fell kindly to Freeman, who gleefully lashed home from close-range.
Bramall lane duly erupted and the home fans were almost celebrating again in stoppage time when Smith saved well before Stefan Scougall lashed the follow-up effort onto the crossbar.
That mattered little, though, because the home side encountered few problems in seeing the game out and securing a priceless victory in their push for promotion.
United: Moore, Fleck, O'Connell, Basham (Scougall 69), Ebanks-Landell, Coutts, Freeman, Duffy, Lafferty (Done 80), Clarke (Lavery 59), Sharp (c)
Subs not used: Ramsdale, Wright, Brown, Reed
Cobblers: Smith, Phillips, Zakuani, Nyatanga, Buchanan, Hoskins (Iaciofano 90), O'Toole, Taylor, Anderson (Potter 69), Revell, Richards (Beautyman 87)
Subs not used: Cornell, Diamond, McCourt, Gorre
Referee: Darren Drysdale
Cobblers fans: 1,650