Weakened Saracens staged a stunning comeback to win 27-25 at Stadium MK and put a big dent in Saints' hopes of a European Champions Cup place for next season.
Saracens looked certain losers as they trailed 25-12 with 10 minutes remaining but two late tries and a penalty and conversion from Alex Lozowski broke Saints' hearts, who had been in control for most of the match.
Saints' bonus point did move them up to sixth place in the Aviva Premiership with two games remaining - their final one being a home fixture against their main challengers for a European spot, Harlequins, who sit one point behind them in seventh.
Saints overcame the injury losses of Louis Picamoles and Stephen Myler to score three tries through Harry Mallinder, Ken Pisi and Dylan Hartley. Myler kicked two penalties and a conversion, with Mallinder also adding a conversion.
Chris Ashton and Marcelo Bosch scored Saracens' tries, with Lozowski kicking five penalties and the match-winning conversion a minute from time. Saracens rested a number of players ahead of next Saturday's European Champions Cup semi-final against Munster.
Owen Farrell, Maro Itoje, Mako Vunipola, Brad Barritt and Richard Wigglesworth were all absentees but George Kruis was back in the starting line-up for the first time since being injured in December. With their first attack, Saints took the lead with a seventh-minute try.
Pisi out-jumped Alex Goode to win possession of a Myler up and under before the wing sent Mallinder on a 30-metre run to the line. Buoyed by this early success, the hosts went on to dominate the opening quarter by battering their opponents' line.
They should have extended their lead by scoring a second try, such was their pressure, but they had to be content with a Myler penalty after Duncan Taylor was yellow-carded for a deliberate off-side.
Despite going down to 14 men, Saracens got their first points after 23 minutes when Lozowski kicked a straightforward penalty as the home side failed to capitalise on Taylor's absence, with the loss of Picamoles to injury a huge blow.
England international Teimana Harrison replaced the Frenchman before Myler kicked his second penalty immediately prior to the return of Taylor from the sin-bin.
A further penalty from Lozowski reduced the arrears before Mallinder, who had taken over kicking duties from a hobbling Myler, hit a post with a 25-metre penalty attempt. But despite this miss, Saints still held a deserved 13-6 half-time advantage.
Myler was unable to return for the second half but within 70 seconds Saints went further ahead when Hartley drove over from close range with Mallinder's conversion leaving the visitors firmly on the back foot.
Lozowski kicked two penalties to bring his side back into contention before Ashton was introduced to replace Sean Maitland, but Northampton responded with the best try of the game with excellent handling creating a try for Pisi.
With 25 minutes remaining, Saracens brought on Billy Vunipola and Michael Rhodes in an attempt to reverse their fortunes and it paid dividends. First Ashton sneaked over in the corner and then Lozowski kicked another penalty before the outside half made the crucial break to set up the winner for Bosch.