Scorers: Northampton Saints: Tries: Tom Stephenson, Kahn Fotuali'i, Luther Burrell, George North 2; Cons: Will Hooley 3, James Wilson
Leinster: Tries: Cian Kelleher, Dan Leavy, Michael McGrath; Cons: Cathal Marsh 3
Second half tries from Cian Kelleher, Dan Leavy and Michael McGrath brought Matt O'Connor's men back to within five points of the hosts, but George North's late effort - his second of the afternoon - took the game away.
A youthful Leinster team displayed their powers of recovery as they bounced back from a 26-0 deficit to finish with a second half flourish.
The bench played its part as full-back Kelleher, Cathal Marsh, who kicked three conversions, and their fellow replacements added plenty to the mix at Franklin's Gardens.
Northampton were able to secure the win thanks to Tom Stephenson, Samoan star Kahn Fotuali'i, Luther Burrell and North, but Leinster were far from overawed.
Instead, they showed their steel, giving the coaches plenty of encouragement and displaying a desire to fight for first team places this season.
The visitors had held their own early on, but some wayward lineouts proved costly in the attempts to build attacking momentum. Northampton eventually relieved the pressure, wrestling back possession, which they used well as England Under-20 centre Stephenson burst through to score. Out-half Will Hooley added the extras.
Leinster went looking for an immediate response and Leavy carried the fight through the middle before a knock on put an end to a promising move.
Northampton were soon back in the groove at the other end though and Fotuali'i was the man on the mark, latching onto an inside pass to score.
O'Connor's charges were a man down soon after as second row Ben Marshall was sent to the sin-bin for a late tackle, bringing Sean McCarthy into the action for his first appearance in Leinster colours. But the men in blue held firm, relieving some pressure by winning a penalty that was to move them from their own 22 into Northampton's.
Some good tackling from Darragh Fanning and a decent take from the high ball by Billy Dardis stopped Saints getting out of their own half as Leinster pushed on. However, the pressure was not maintained for long and Northampton registered their third converted try as star man Fotuali'i broke free and set up Burrell to score and make it 21-0.
Leinster tried to issue a response, with stand-in captain Jimmy Gopperth pulling the strings and Brendan Macken using his brawn to earn a penalty. Gopperth had the line in sight as he found a gap, but his charge was halted by a big tackle from Ben Foden.
Both sides made changes at the break, with Gopperth among those replaced by Leinster. Northampton were quick to add to their tally as Wales winger North, on at the break, wasted no time in easing in for an unconverted try out wide, making the most of good midfield work.
Leinster quickly introduced more fresh faces and one of them, Kelleher, made the most of great work by Marsh and McGrath to register a try. Marsh added the extras with a sweetly struck conversion and Leinster were now on top.
They were soon a man up, too, as Northampton's replacement scrum half Lee Dickson was sent to the sin-bin for a dangerous tackle on Steve Crosbie. And Leinster profited immediately, flanker Leavy (pictured below) claiming a try that was converted by the impressive Marsh.
It was a stirring fightback from O'Connor's side and it was to get even more impressive before the end as Clontarf ace McGrath found his way over.
Marsh's successful conversion meant Leinster were back to within five points at 26-21, but any hopes of a stunning victory were dashed as North picked up a pass from Cam Dolan and flew in to score.
James Wilson added the conversion to put the finishing touches on a win that Northampton were made to work hard for.
NORTHAMPTON SAINTS: Ben Foden; Jamie Elliott, Dom Waldouck, Tom Stephenson, Ken Pisi; Will Hooley, Kahn Fotuali'i; Alex Corbisiero, Ross McMillan, Salesi Ma'afu, James Craig, Christian Day, Phil Dowson, Tom Wood (capt), Samu Manoa.
Replacements: Dylan Hartley, Alex Waller, Gareth Denman, Courtney Lawes, Calum Clark, Jon Fisher, Cam Dolan, Lee Dickson, Sam Olver, Luther Burrell, George Pisi, George North, James Wilson.
LEINSTER: Billy Dardis; Darragh Fanning, Brendan Macken, Jordan Coghlan, Sam Coghlan Murray; Jimmy Gopperth (capt), Isaac Boss; Michael Bent, Bryan Byrne, Tadhg Furlong, Ben Marshall, Tom Denton, Josh van der Flier, Dan Leavy, Dominic Ryan.
Replacements used: Sean McCarthy for Bent (32 mins), Gavin Thornbury for Marshall, Luke McGrath for Boss, Cathal Marsh for Gopperth (all half-time), Michael McGrath for Coghlan Murray, Steve Crosbie for Coghlan, Cian Kelleher for Dardis (all 50), Rory O'Loughlin for Macken, Adam Byrne for Fanning (both 57), Jonathan Slattery for Ryan, Tyrone Moran for B Byrne (both 62), Ed Byrne for McCarthy (68).
It will be an all-Irish affair at the GUINNESS PRO12 final on Saturday as, after 22 rounds of tough competition, the two teams that finished top of the table after the regular season - Leinster and Connacht - will face each other at the iconic BT Murrayfield.