Tries from Roger Wilson and Ben Foden, the latter putting in an excellent performance in front of watching England coach Stuart Lancaster, along with 14 points from Ryan Lamb was enough to see off the Aviva Premiership Rugby table toppers.
Saints started the game on the back foot, so much so that in the opening 10 minutes the sum total of their time in possession was just 35 seconds. Nevertheless they were first on the scoreboard through a penalty from Lamb and it might have got even better for the hosts soon after but Phil Dowson lost the ball when heading for the line.
The men in green, black and gold then began to put heavy pressure on the Quins' line and after referee Dave Pearson had issued repeat warnings to the visitors' defence, Nick Easter was yellow carded. Saints went repeatedly for the corner, but just as they looked to be struggling to turn that pressure into points Lamb slotted a delightful and well-timed drop goal through the uprights to double the lead.
With their numerical disadvantage, Quins were looking like a side on the rack and on 26 minutes the Saints struck a vital blow. As the ball was spun out through the backs, Lamb did well to hold on after a juggle and Foden was on hand to beat Mike Brown, the contender for the Saints man's England shirt, and score in the right-hand corner in front of a delighted Burrda Sport Stand.
The Saints were playing with poise and purpose so it was no surprise when Lamb slotted his second penalty on 32 minutes to extend the lead to 14 points but the visitors soon replied with their first points when Rory Clegg kicked his first penalty of the night.
Quins rallied with some pressure late in the first half but it was Lamb who extended the lead with the last kick of the first 40 minutes, his third penalty sending the Saints into the half-time interval with a well-deserved 17-3 lead.
As they had with the first half Quins started the second period on the front foot and putting Saints under considerable pressure. This time their play yielded penalties, but while Clegg went for goal his radar was well off and he pulled two shots at the sticks left of the uprights. It wasn't long before the Saints made the fly half pay for his misses. After spinning the ball from left to right and then back again, Roger Wilson stopped an opening in the Quins defence and scythed through to score the Saints' second try.
It was the perfect tonic for the sold-out Gardens crowd, but as the visitors charged upfield from the restart, Mark Sorenson was sent to the sin bin by referee Pearson for coming in at the side of a ruck. Despite being down to 14 men for 10 minutes, the Saints did what the Aviva Premiership Rugby leaders could not do earlier on and keep their line intact.
Despite some well-constructed moves from both sides in the remaining quarter of the match, including some fantastic runs from the impressive Foden, neither could find another score and the Saints were left celebrating a superb win over the league leaders that consolidated their position in the play-off places going into the next four weeks of action in the Heineken and LV= Cups.
REFEREE Dave PEARSON
AVIVA MAN OF THE MATCH Samu MANOA
13mins SAINTS Pen Lamb 3-0
20mins QUINS Yellow Card Easter
22mins SAINTS Drop-goal Lamb 6-0
26mins SAINTS Try Foden 11-0
32mins SAINTS Pen Lamb 14-0
34mins QUINS Pen Clegg 14-3
40mins SAINTS Pen Lamb 17-3
HALF-TIME SAINTS 17 HARLEQUINS 3
56mins SAINTS Try Wilson Con Lamb 24-3
58mins SAINTS Yellow Card Sorenson
FULL-TIME SAINTS 24 HARLEQUINS 3