Saints produced a stirring second-half showing to see off Saracens 25-20 and cement top spot in the Aviva Premiership.
Six Stephen Myler penalties and a converted Samu Manoa score sealed a vital victory at stadium:mk.
Saracens, who are now nine points behind the table-toppers, had been in control early on through two Chris Wyles tries.
The visitors led 17-13 at the break, but their scrum fell apart in the second and Maro Itoje’s sin-binning put them under pressure.
Saints eventually had too much power and Myler booted them to victory, to the delight of more than 27,000 fans at a ground awash with green, black and gold.
Jim Mallinder’s men had headed into the game on the back of sizeable defeats at Clermont Auvergne and Exeter Chiefs.
And they were to start slowly again, with wing Wyles wandering in for the opening try.
Charlie Hodgson’s conversion meant the visitors, backed by a small section of the crowd, enjoyed a 7-0 lead inside six minutes.
But Myler, who failed to register a point in the previous two games, cut the gap almost immediately with his first penalty attempt of the afternoon.
And he was on target again soon after, but Saracens were finding it easy to break through at the other end after Saints got turned over five metres from their line, Wyles scored again.
Saints had been torn apart twice in as many attacks and Hodgson’s accurate conversion from the touchline rubbed salt in the wounds.
The men in white and red, which Northampton were wearing in support for the Help For Heroes charity, needed a lift and Luther Burrell was the man to provide it.
The England centre’s scything run put Saracens in trouble and American star Manoa picked up the pass to run in for a simple score.
Myler’s conversion cut the gap to a single point, but Saints were struggling in the scrum, with Saracens squeezing penalties from the set piece.
However, Hodgson’s kick went to the left of the uprights before Myler was similarly wayward with an effort at the other end.
And the Saints fly-half was soon breathing a huge sigh of relief as Saracens No. 8 Billy Vunipola missed a huge chance after charging down Myler’s kick.
Under no pressure, Vunipola went to ground and as he bounced towards the line, the ball came out of his hands and was knocked on.
It was a massive reprieve for Saints, but Hodgson wasn’t about to let them off the hook with a penalty soon after, extending his side’s lead to four points before the break.
It was all Saints at the start of the second half as Saracens, who had changed into their black home shirt, which arrived at half-time, were forced into some stubborn resistance.
And having survived the waves of pressure, the away side soon had another three points to their name as Hodgson kicked well from close to halfway.
Saints needed a boost and lofty lock Itoje’s sin-binning gave them a platform to build on.
Ken Pisi came within inches of a score as he was held up over the line before some Harlem Globetrotters-like play from Tom Wood and Myler failed to yield reward.
The following scrum did though, as Saracens conceded a penalty, which Myler landed with ease.
And he soon had another three points to his name as Saints continued to turn the screw.
Saracens were struggling to get out, and they were having real problems at the scrum as they surrended another penalty on their own put-in.
Myler elicited a huge roar from the crowd as he planted the ball between the posts to put his team ahead for the first time at 22-20.
His sixth successful kick seven minutes from time meant Saracens needed a converted try to win, but, with their scrum continually creaking, they were struggling to muster an attack.
As the clock hit 80, Myler had a chance to put gloss on the performance, but his kick hit the post, with Saracens trying to mount a charge from their own line.
But Saints simply shut the door and suffocated their opponents to secure a hugely significant success in the race for a home play-off semi-final.
Saints: Tuala (Wilson 65); K Pisi, G Pisi, Burrell, Elliott (Stephenson 74); Myler, Fotuali’i (Dickson 55); Corbisiero (A Waller 61), Hartley (c) (Haywood 69), Ma’afu (Denman 48); Lawes (Dickinson 74), Day; Wood, Clark, Manoa (Fisher 74).
Saracens: Goode; Strettle (Ashton 61), Bosch, Barritt (c), Wyles; Hodgson (Farrell 55), de Kock (Wigglesworth 51); M Vunipola (Gill 65), Brits, Johnston (Longbottom 55); Kruis, Itoje; Joubert, Brown, B Vunipola (Wray 39 (Hamilton 40)).
Referee: Greg Garner