Saints kept their European dream alive with a superb 22-10 success at Scarlets on Saturday night.
Harry Mallinder scored his second try in as many Champions Cup matches to get them up and running before efforts from George Pisi, Sam Dickinson and George North secured the vital bonus point.
Scarlets did score a late try through replacement prop Rhodri Jones, but it mattered not as Jim Mallinder’s men had already got what they were looking for.
Saints, who have finished second in Pool 3, will now have to wait until Sunday to see whether they can claim a best runners-up spot to make the quarter-finals.
They will need a favour from either Exeter Chiefs, who play Ospreys at Sandy Park, or Leicester Tigers, who play at Stade Francais, if they are to make it.
But Saints at least did their job in Wales as they produced a display packed with style and desire in at a rain-soaked Parc y Scarlets.
The away side started with an adventurous style and Stephen Myler was lucky as Tom Williams knocked on while intercepting a long, lofted pass in his own half.
But when Scarlets were guilty of showing the same kind of ambition soon after, Mallinder made them pay, lapping up a wayward Gareth Davies pass and sprinting clear to score.
Myler converted with ease and Saints had exactly the start they craved.
it was to get even better just past the 20-minute mark as a slick attacking move cut Scarlets apart and Samoan star Pisi slid in for a try in the corner.
Myler missed the conversion, but it didn’t matter too much for Saints, who were halfway to their try target.
Scarlets responded by winning a scrum penalty, with fly-half Aled Thomas getting their first points of the game from the tee.
But Saints were still in control and they had a big chance to add a third try before the break.
After winning a penalty at the scrum, they went for the corner, but eventually the ball was lost at the breakdown and Scarlets cleared their lines.
It meant Saints were halfway there with half of the game gone and they knew they would need another two tries after the break.
And they started their quest well, scoring just seven minutes into the second period as a poor clearance from Scarlets full-back Collins provided a lineout deep in home territory.
From that, Saints drove over, with No.8 Dickinson the man to dot down.
Myler missed the conversion, but Scarlets were suddenly struggling and they saw lock Maselino Paulino yellow carded for a cheap shot on Courtney Lawes.
And Saints soon took advantage to score the fourth try they so badly needed.
Great work from Pisi and Mallinder weaved an opening for North, who gleefully grabbed a try against his former club.
Boos rang out at the sparsely-populated ground as the name of North was read out, but Saints didn’t care as they celebrated.
With Mallinder’s men just needing to hold out for the win, Scarlets eventually started to apply some pressure.
And Saints scrum-half Lee Dickson, on for the impressive Tom Kessell, was sin-binned with seven minutes to go.
Scarlets grabbed a consolation score in the final minute as replacement prop Rhodri Jones burrowed over and Thomas converted.
But it didn’t matter to Saints, who had secured the five points they were looking for.
Scarlets; Collins (Hughes 60); Williams, King, S Shingler, van der Merwe (Lewis 60); Thomas, G Davies (A Davies 55); John (Evans 64), Owens (c) (Myhill 64), Lee (R Jones 46); Price, Paulino; A Shingler (Rawlins 65), Boyde (Phillips 57), Allen.
Saints: Foden; Elliott, G Pisi (Burrell 59), Mallinder, North; Myler (Hanrahan 70), Kessell (Dickson 59); A Waller (E Waller 59), Hartley (c) (Haywood 60), Hill (64); Lawes, Day; Gibson (Fisher 6), Wood, Dickinson (Paterson 64).
Referee: Pascal Gauzere (France)