Jonathan Sexton scored all of Leinster’s points as the 2009 Heineken Cup winners held out for a crucial 25-23 Pool Two victory at Wembley.
The Ireland fly-half was ruthless in front of the posts, kicking six penalties and converting his own second-half try to leave Saracens with two defeats from two.
The Watford-based side kicked six penalties of their own, two from Derick Hougaard and four from Alex Goode, who also crossed for an early try for the hosts.
Sarries had been beaten 25-10 in their opening clash at Clermont Auvergne but were hoping their 100 per cent Wembley record would continue – having played four times at the ground last season and won four, including a memorable 24-23 victory over world champions South Africa.
Leinster failed in their efforts to get Ireland captain Brian O'Driscoll fit for action. The Lions centre failed a fitness test on a troublesome hamstring .
His place went to another 2009 Lion, Luke Fitzgerald, forming a dangerous midfield pairing with Gordon D'Arcy as Leinster made their Wembley debut.
Saracens lost their first line-out, a costly error that saw them under pressure and back-tracking into their 22 until a lengthy clearance kick by fly-half Hougaard relived the situation.
But back came the Irish province, and this time referee Christophe Berdos penalised Saracens for not releasing at the breakdown.
Sexton opened the scoring on seven minutes, his penalty clipping the crossbar before dropping over.
Hougaard then hit a post with a ninth-minute penalty, and when Saracens were awarded another on halfway he went for touch, and his confidence was rewarded with the game's first try on 12 minutes.
Three phases, a fine drive by Andy Saull and Goode sold a dummy pass towards David Strettle, bought by the Leinster defence, before scoring on the right.
Hougaard missed the conversion, but kicked a confident penalty four minutes later from 30 metres.
The referee went upstairs when Cian Healy crossed the Saracens line after a sweeping attack,
and the video official spotted a knock-on.
But Leinster kept the pressure on and Sexton kicked his second penalty in the 21st minute.
Saracens were again guilty of hanging on to the ball too long at the breakdown, gifting Leinster a penalty in front of the posts which Sexton drilled home with 27 minutes on the clock.
Saracens managed to exert some pressure of their own, winning a scrum, shunting Leinster and earning a penalty on 36 minutes. Hougaard made no mistake to recapture the lead.
But no sooner had the cheers died down than referee Berdos awarded Leinster a penalty for offside and Sexton boomed it over from 45 metres.
Sexton punished Saracens yet again five minutes into the second half, firing over his fifth kick to extend Leinster's lead.
To make matters worse for Sarries, Hougaard pulled up suddenly, fell to the ground and clutched the back of his right leg. The South African departed the fray with USA wing Chris Wyles switching to full-back and Goode moving to fly-half.
The disruption did Saracens no favours, and it was a missed tackle by Strettle on Isa Nacewa up the left that led to Leinster strengthening their grip. Nacewa's pass to D'Arcy saw the centre draw his man before sending Sexton galloping in for the score which he also converted.
Goode rallied Saracens huge following in the 45,892 crowd with a penalty on 56 minutes, but a rash of replacements only served to further disrupt the hosts, whose Heineken Cup campaign was looking in serious danger of derailing early.
So it was their first glimmer of hope when Leinster replacement Richardt Strauss was sin-binned for persistent infringements – and Goode stepped up to steer the penalty home and boost the crowd.
But Schalk Brits was then pinged for a soft penalty which the superb Sexton kicked without fuss, only for Goode to keep Saracens hopes alive with his third successful kick.
And with six minutes to go, Goode punished Leinster's indiscipline at a scrum, scoring his fourth penalty to reduce the deficit to two points.
Saracens put together an astonishing 30 phases as they went in search of a last-gasp drop-goal but Leinster held out with a brilliantly disciplined defensive effort.