Tries from Ken Pisi and Alex Waller helped Saints secure a nerve-shredding 15-11 Champions Cup win against Scarlets at Franklin’s Gardens on Saturday night.
Jim Mallinder’s men were made to battle all the way by the Welsh side, who were eventually left to rue four missed kicks.
Scarlets failed to take their chances, especially during sin-bin periods, with George North yellow carded in the first half and Jamie Gibson in the second.
Saints showed great spirit to see those spells out and they were eventually to celebrate the victory in the first match of Pool 3, which also contains Glasgow Warriors and Racing 92.
Ken Pisi gave them the perfect start and Alex Waller made the most of the home side’s impressive forward dominance in the second period to cement the victory.
Saints now head to Glasgow next Saturday as they bid to lay a marker down in the bid to make the quarter-finals.
They had gone into the game against Scarlets having lost three of their first four Aviva Premiership matches, while Scarlets had won six of their seven in the Guinness Pro12.
But Saints looked a different side from the one that slugged it out in a defeat to Saracens at the Gardens a week earlier, starting with real vibrancy.
And after JJ Hanrahan spotted a gap in the Scarlets defence, Ken Pisi latched onto the Irishman’s pass to score.
Hanrahan added the extras and Saints had the sort of start they craved in the wake of the stumble against Sarries.
But Scarlets responded strongly, with a surge from centre Gareth Owen stopped at the last before Steven Shingler landed a penalty.
And the visiting fly-half was soon lining up another penalty, after a handling error from Ken Pisi cost Saints, but this time Shingler pushed his kick wide.
He looked likely to regret that miss as Saints appeared to have scored a superb team try.
Alex Waller turned the ball over, Ken Pisi broke and the ball was worked superbly to Luther Burrell, who dived over in the corner.
But referee Romain Poite saw the replay on the big screen and noticed that North had stamped on Michael Tagicakibau in the build-up.
The try was disallowed and North, facing his former club, was sent to the bin in a disaastrous episode for Saints.
But the home side soon had a shot at goal as Tom Wood did brilliantly at the breakdown and some Scarlets indiscipline saw the penalty moved forward.
Hanrahan fancied it, but fired his long-range effort wide, and he soon missed another after a penalty was given for Jake Ball’s late tackle on Lee Dickson.
Saints had totally bossed the sin-binning period and they were to close to scoring in the final seconds before North’s return.
A huge drive, which even included Burrell and George Pisi, pushed Scarlets back, but the ball was held up and Saints took the five-metre scrum.
However, the home team failed to find their way over from the resulting move, lacking a finishing touch after all of their efforts.
Saints were winning the breakdown battle, but their handling was letting them down and after a knock on on the edge of the home 22, Scarlets seized a penalty chance.
Shingler cut the gap to a single point before the break and his team were growing into the game.
The fly-half had the chance to give his team the lead for the first time two minutes after the restart, but his penalty was horribly skewed.
But Scarlets soon made up for that miss as a poor Dickson box kick put Saints in trouble and Scarlets finished superbly, with scrum-half Gareth Davies darting over.
The boos rang out as Poite refused to review what looked to be a forward pass in the build-up, and Shingler hit the post with the conversion.
Saints responded by turning up the heat in the maul, an area they had controlled all night, and Alex Waller profited to restore his team’s one-point lead.
Hanrahan missed the conversion and Saints were then hit by another sin-binning, with Gibson seeing yellow for not releasing.
But as they had when North was forced to spend 10 minutes on the sidelines earlier, Mallinder’s men showed real spirit.
They won a penalty at the scrum thanks to good work from Paul Hill, and Hanrahan did the rest to give his team a four-point lead.
Aled Thomas missed the chance to narrow the gap to one as the full-back’s penalty came back off the right upright, and Gibson returned to boost his team with five minutes to go.
Scarlets gave it everything they had, but they were to just fall short, eventually opting to boot the ball out to take the losing bonus point, giving Saints a vital success.
Saints: Foden; K Pisi (Tuala 53), G Pisi (Myler 61), Burrell, North; Hanrahan, Dickson (c) (Fotuali’i 72); A Waller (E Waller 64), Haywood, Brookes (Hill 64); Craig (Harrison 65), Day (Paterson 20); Gibson, Wood, Dickinson.
Replacements not used: Williams.
Scarlets: Thomas; Tagicakibau, Owen, Parkes, van der Merwe; S Shingler (Williams 74), G Davies (A Davies 74); Evans (John 66), Phillips, Edwards; Ball, Paulino (Rawlins 57); A Shingler, J Davies, Barclay (c).
Replacements not used: Myhill, Taylor, Price, King.
Referee: Romain Poite (France)