Three first-half penalties from Stephen Myler edged the Saints ahead before their territorial dominance was rewarded with Paul Diggin's interception try. Leinster rallied to score two late tries, one with the last play of the game, but the Saints were already home and hosed.
The opening 10 minutes were slowed up somewhat by a series of failed scrums, but when the ball finally got moving, it was the Saints and James Downey who took the initiative. The centre's line break was strong and with Leinster offending at the breakdown, Myler gave the Saints an easy three-point start with the penalty.
It was a lead almost extended, Myler finding Diggin on a beautiful line, but Leinster defenders scrambled well enough to drag Noah Cato, the furthest man left, through the corner flag and into touch.
The home side responded, but a series of drives close to the Saints line couldn't quite find the thrust they needed and the Saints moved quickly upfield before doubling their lead. It was a fast and incisive move, highlighted by a lovely offload from the falling Ben Nutley and the offence came when Leinster hands went into a ruck.
A third kickable chance soon followed, but when the decision to go to the corner failed to yield any points, next time round Myler pointed to the posts and continued to strike the ball well as the Saints moved 9 points clear.
The Saints had been camped out in Leinster territory for a good 15-minute spell and when the hosts got their chance to attack in the half's final play, they were again met by a wall of Saints white and a knock on brought about the interval.
If Leinster were hopeful the break would slow the Saints down, they were sadly mistaken as the momentum continued into the early stages of the second-half. The game would have had its first try had the ball not squirmed from Diggin's grasp on the line. Not that the winger was to be denied, though it was from an interception that he would score, reading the play well as Leinster looked to run it clear.
Lamb converted to make it 16-0 just past the hour, but Leinster weren't to be deterred from playing riskily and giant number 8 Leo Auva'a showed his dancing feet to bring the crowd to their feet and almost put Fionn Carr in from his own 22. The move broke down but next time the home faithful did have something to cheer as a huge maul powered its way over the Saints line seven minutes from time.
Unfortunately from their point of view it wasn't to be the start of a dramatic comeback as Ryan Lamb thumped over a long penalty, but they would score the final points of the night as Carr added a second try. It only acted to decrease a margin of defeat the Saints more than deserved and Jim Mallinder's men closed their pre-season in the best possible way.
REFEREE George Clancy
11mins SAINTS Pen Myler 0-3
26mins SAINTS Pen Myler 0-6
36mins SAINTS Pen Myler 0-9
HALF-TIME LEINSTER 0 SAINTS 9
62mins SAINTS Try Diggin Con Lamb 0-16
73mins LEIN Try Harris-Wright Con Berquist 7-16
79mins SAINTS Pen Lamb 7-19
80mins LEIN Try Carr Con Berquist 14-19
FULL-TIME LEINSTER 14 SAINTS 19