The Welsh side needed to win to stay in the hunt for a best runners-up spot and had the better of the first half as a try from wing Vili Iongi plus nine points from fly-half Rhys Priestland helped them to a 14-6 interval lead.
But Northampton's renowned forward power changed the complexion of the game after the break as tries from Soane Tonga'uiha and Ben Foden, supplemented by Stephen Myler's 17-point haul, saw them secure a fifth win in a row in all competitions.
Much of the pre-match focus had been on referee Alain Rolland, who was taking charge of a game in Wales for the first time since sending Sam Warburton off in the national side's World Cup semi-final defeat to France.
The Irishman's name was roundly booed prior to kick-off, and he received further jeers as he took to the field, but it did not take him long to get back in credit with the home support as he sin-binned Saints centre Tom May for childishly throwing the ball at Gareth Davies after being called back for a forward pass.
Scarlets number eight Ben Morgan, named in England's RBS 6 Nations squad earlier this week, laid down an early marker in his battle with Red Rose back-row colleagues Phil Dowson and Calum Clark with two punishing early carries and strong defensive work as the hosts opened strongly.
Saints' ill-discipline allowed Priestland to open a 6-0 lead with two penalties, before Myler responded with a three-pointer of his own after Chris Ashton and Foden had trapped Liam Williams in his 22.
Priestland's third penalty arrived after Rolland reversed a call against the Scarlets for Clark carrying on after the whistle had gone, but the visitors were starting to turn the screw.
Breaks from Martin Roberts and Ashton sparked continuous Saints pressure but twice the Scarlets escaped desperate defensive situations, with May being held up over the line in the first of those instances.
Northampton's inability to make the most of those opportunities proved costly as the Scarlets grabbed the first try after 32 minutes.
Jonathan Davies battered back against the angle to lay the platform before Priestland dropped a superbly-judged cross-kick on Iongi for the winger to dot down.
Myler trimmed the lead to 14-6 with his second penalty, although the Scarlets could have taken a greater advantage into the break had George North, a prominent figure throughout, not thrown a speculative pass to a defender after some typical strong running.
But the Scarlets' good work was quickly undone as Northampton battled their way into the lead within nine minutes of the restart.
Video referee Seamus Flannery had already turned away an effort from Clark after a Saints maul had rumbled over the Scarlets line, but he was more forthcoming when it came to Tonga'uiha's close-range grounding.
Myler added the conversion before slotting his third penalty after a scrappy passage of play to give Northampton the lead for the first time.
Priestland responded with a penalty of his own, but the composure the Scarlets had shown in the first 40 minutes had briefly deserted them as Northampton's forward power took hold at the breakdown and the loose, with two further three-pointers from Myler putting the visitors 22-17 to the good.
The home side threw on the experience of Stephen Jones in an effort to wrest control back in their favour, but with their Heineken Cup hopes hanging by a thread the Scarlets were forced into some desperate efforts to attack from deep and after flirting with disaster one such attack almost produced a stunning score from distance.
North raced up to halfway before Morgan and Lou Reed took the attack on, but scrum-half Gareth Davies ignored a gaping overlap and the chance was lost.
Chances continued to come for the hosts in a barnstorming finish, with Foden's brilliant cover tackle denying Liam Williams in the corner.
And the England full-back made another telling intervention to finally take the game from the Scarlets' grasp as, with Northampton camped on their own line, he picked off Rhodri Williams' pass to go the length of the field.
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