The Irish side inflicted heavy defeats on the black, green and gold in Northampton in December 2013 and December 2016 - and they were at it again on Saturday afternoon.
Leinster proved too powerful and too streetwise, scoring seven tries as they earned a 43-16 success and the maximum haul that puts them in control of Pool 1.
They are now five points ahead of second-placed Saints, who must go to the Aviva Stadium to face Leinster again next Saturday evening.
Chris Boyd's men had briefly threatened to travel to Dublin with a win under their belts after taking a 16-14 lead during the first period.
Ahsee Tuala finished off a flowing move and Dan Biggar landed three penalties and the conversion.
But Leinster kept coming back and the crucial period came just after half-time as the Irish giants scored their fourth and fifth tries of the match inside a ruthless three-minute spell.
Saints tried to summon a response and did enjoy plenty of possession, but Leinster's defence was determined as they preserved their unbeaten start to the season.
Saints endured a nightmare start to the match as Cobus Reinach's pass went astray in the Leinster 22 and the away side immediately pounced, Jordan Larmour breaking before releasing James Lowe for the score.
Sexton converted to make it 7-0 inside just three minutes and Saints needed to issue an immediate riposte.
They did just that as Teimana Harrison delivered a brutal carry and the ball was eventually moved left at real speed for Tuala to dive over in the corner.
Biggar converted superbly from the left touchline to level the scores, and the noise levels were beginning to rise.
Saints were starting to get into their groove and they were so unlucky not to score again as Reinach put a grubber through and the bounce beat Tom Collins.
A penalty was coming though, and Biggar landed it to make it 10-7 in the home side's favour.
Leinster replied almost immediately, despite a sensational try-saving tackle from Reinach on fellow scrum-half Jamison Gibson-Park.
The ball was recycled and patient play allowed Rhys Ruddock to power over, with Sexton converting to make it 14-10 in a frantic encounter.
Saints were still coming at Leinster, using their scrum as a platform, and Biggar was able to land a penalty to cut the gap to one point after a big carry from Alex Waller.
And Saints were in front on the half-hour mark after some more high-tempo play put Leinster on the ropes, allowing Biggar to slot a penalty.
But Leinster landed a blow before the break, kicking to the corner and eventually rumbling over through tighthead prop Andrew Porter.
Sexton sliced the conversion, sending it wide of the right post, leaving the score at 19-16 at the end of an electrifying first 40 minutes.
But Leinster soon extended their lead after the restart, picking up the bonus point as once again the forwards cranked up the heat and Cian Healy finished well.
Sexton limped off and his replacement, Ross Byrne, did the business from the tee to make it 26-16.
Leinster scored again soon after, Byrne skating in for his team's fifth try as Saints splintered and Waller saw yellow.
Byrne hit the post with the conversion to leave the score at 31-16 with Saints needing to scale a mountain.
They came back with bite, but Leinster's defence stood tall on their own line to add to the frustration.
Garry Ringrose was sin-binned for a tip tackle on Biggar to bring the teams level in terms of personnel and Saints attempted to start their comeback.
But Taqele Naiyaravoro knocked on close to the Leinster line and the Irish giants breathed again.
Saints, to their credit, kept coming forward, but they were repelled time after time by the blue machine.
And Leinster had the final say as replacements Luke McGrath and Ed Bryne both dotted down to put the seal on a resounding success.
Saints: Tuala (Francis 58); Collins, Proctor, Hutchinson, Naiyaravoro; Biggar (Grayson 63), Reinach (Tupai 66); Waller (cc) (van Wyk 58), Haywood (van Vuuren 60), Painter (Hill 50); Moon, Ratuniyarawa; Wood (Ludlam 50), Gibson (Coles 63), Harrison (cc).
Leinster: Larmour; D Kearney, Ringrose, Henshaw (R Kearney 78), Lowe; Sexton (c) (R Byrne 45), Gibson-Park (McGrath 54); Healy (E Byrne 54), Kelleher (Tracy 54), Porter (Furlong 54); Toney (Fardy 56), Ryan; Ruddock, van der Flier, Doris.
Referee: Alexandre Ruiz