The visitors had forwards Calum Clark and Phil Dowson sin-binned midway through the second half and Sale took advantage by overpowering Jim Mallinder's men.
All Northampton's points came from the boot of fly-half Stephen Myler, but it was not enough to prevent a third straight defeat.
Myler and opposite number Nick MacLeod both missed penalties during the early exchanges before Myler made it second time lucky with an eighth-minute penalty to put Saints ahead.
Yet the momentum quickly shifted in Sale's favour as they attacked the visitors with aggression and pace.
Sharks centre Andrew Higgins embarked on a searing run inside the right channel, driving into the heart of Northampton's 22-metre line before his progress was halted.
It was one of several assaults on the Saints defence by the Sale backline but they could not make it pay.
Northampton withstood the early pressure and Myler booted his second penalty in the 16th minute to double his side's advantage.
He added his third from the halfway line three minutes later before Northampton were again forced to repel an onslaught from the home side, for whom Peel and MacLeod again dovetailed effectively at half-back.
Sale's pressure told in the 20th minute when MacLeod kicked a penalty to get them off the mark but Northampton then enjoyed a purple patch which threatened to take them over the line.
With Northampton camped on the edge of Sale's line, Peel incurred the wrath of referee Tim Wigglesworth and was sin-binned for kicking the ball out of Martin Roberts' hands as he attempted to take a quick penalty tap.
Yet to Sale's credit, they repelled wave after wave of Northampton pressure on their line.
Although Myler landed his fourth penalty from five attempts in the 36th minute, MacLeod struck his second moments later to leave the scores finely poised at 12-6 in Northampton's favour.
Four minutes after the restart, Myler landed his fifth penalty and MacLeod then hit back with his third as the boot dominated once again as Myler kicked his sixth penalty in the 54th minute.
Four minutes later, Sale were rewarded for a period of sustained pressure as Easter grounded the ball in the left corner after Northampton finally buckled.
MacLeod could not add the extras but after Saints had Clark sin-binned in the 59th minute and then Dowson was shown a yellow card three minutes later, Sale were playing against 13 men.
A penalty from MacLeod cut the gap to one point but four minutes later Peel scampered over in the right corner for a try which MacLeod could not convert.
Myler then kicked his seventh penalty to cut Sale's lead but a fine score from Seymour in the 74th minute proved decisive as he burst clear inside the left channel and MacLeod added the extras to deny Northampton a losing bonus point.
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