Saints landed an early shot across the bows of Heineken Cup opponents Leinster with a dogged 21-13 win at Donnybrook Stadium on Friday night.
But the margin of victory could have been far greater as Jim Mallinder’s men, playing their final pre-season game, had plenty of chances to punish their hosts.
The victory was secured thanks to tries from prop Gareth Denman and Samu Manoa, along with five points from the boot of Stephen Myler and six from young fly-half Sam Olver.
Leinster, without any of their Ireland and British & Irish Lions stars, had held an 8-5 lead at the break as their determined defence paid dividends.
But Saints were simply too strong during the second half as they eventually made their possession pay.
And tougher tests now lie in wait as the Premiership season begins next Saturday, with Exeter the visitors at Franklin’s Gardens.
With that game in mind, Saints named a strong starting 15 for the clash in Dublin, with Lions stars George North and Alex Corbisiero making their first starts in the green, black and gold.
USA star Manoa also came in for his action of the friendly campaign, while, in contrast, Amlin Challenge Cup and RaboDirect PRO12 champions Leinster named a scratch side.
Bearing that in mind, Saints, who had won all three of their pre-season encounters to date, were expected to boss things from the off.
But they didn’t have it all their own way early on as their hosts showed plenty of aggression at the breakdown to stem the lime tide.
And Leinster were the ones to get the scoring started, shoving their way towards the line with a mammoth lineout drive, with prop Martin Moore the grateful beneficiary.
Former Newcastle Falcons fly-half Jimmy Gopperth missed the conversion from the right, but the home side boasted a 5-0 lead with six minutes gone.
The try gave Saints a shot in the arm and though they nearly shot themselves in the foot when a George Pisi pass was intercepted, the away side were in the ascendancy.
Two penalties were kicked to the corner in quick succession and the men in lime finally had their try as Leinster were given a taste of their own medicine with a lineout drive.
Denman was the man to get the score, getting his second try of the pre-season campaign, but Myler’s conversion was just wide of the right post.
Leinster were soon back in front as Saints were penalised in front of their posts and Gopperth landed a simple penalty as the rain started to fall on Donnybrook.
Saints were finding their Irish opponents more slippery than they might have expected and Lee Dickson summed up the frustration in the away ranks by punching the ball into the ground in anger as the home side were awarded another scrum.
Saints won the ball back, as they had on numerous occasions, but they were drowning in a blue wave of defence and even juggernaut North was struggling to make an impact.
Ken Pisi was denied a try before the break, losing the ball before he could put any pressure on it, and Leinster held a hard-fought 8-5 lead at the interval.
Saints emerged with the bit between their teeth at the start of the second half and they finally turned possession into points as Manoa touched down from another dominant maul.
Myler added the extras and Northampton had the lead they merited.
A raft of replacements had upped the tempo and Leinster were now struggling to cope as Saints set up camp in their 22.
Myler made the most of a penalty in front of the posts to stretch his side’s lead to 15-8 and they continued to pile on the pressure.
Olver, on for Myler, made it 21-8 with two well-struck penalties before Leinster, who had shown little since the break, got a last-gasp consolation through Dan Leavy.
It was too little too late, though, and Saints maintained their momentum against a side they will be reunited with in Europe during the upcoming campaign.
Saints: Wilson (Collins 60); K Pisi (Artemyev 63), G Pisi (Stephenson 44), Burrell (Diggin 48), North (Elliott 50); Myler (Olver 60), L Dickson (Glynn 63); Corbisiero (A Waller 48), McMillan (Haywood 40), Denman (Mercey 40); Lawes (Craig 60), Manoa (van Velze 50); Clark (Wood 50), Dowson (c) (Nutley 60), Dickinson (Day 40).
Leinster: Hudson; Fanning, Macken, Reid, Coughlan-Murray; Gopperth, L McGrath; G McGrath, Dundon, Bent; Roux, Denton; Ruddock, Jennings (c), Murphy.
Replacements: Cronin, Tracy, O’Connell, Moore, Furlong, Beirne, Marshall, Ryan, Leavy, Auva’a, Reddan, Cooney, Marsh, Daly, O’Shea, McFadden, Byrne, Kearney.
Referee: George Clancy