Jonathan Sexton scored all of Leinster's points as the 2009 Heineken Cup winners held out for a crucial 25-23 Pool Two victory over Saracens 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.