The 24-year-old lock, who played all three Tests on the 2017 tour against New Zealand, grabbed the fourth try for the Premiership champions as they booked a home quarter-final and the No.1 seeding.
With the game finely poised at 24-19 to Saracens, Itoje dotted down the pivotal score at Allianz Park with just over ten minutes left on the clock to seal the bonus-point win.
Itoje’s score capped off a strong individual display, which resulted in him being awarded the man of the match award, with fellow Lions Mako Vunipola and Jamie George also starring for the hosts.
Elsewhere, Iain Henderson was man of the match on his return from injury as the Irish forward helped Ulster claim a hard-fought victory against Leicester Tigers at Welford Road.
The British & Irish Lions lock last played for the Irish province in December after suffering a thumb injury which required surgery and was expected to be out until March.
But he showed his powers of recovery to return for Ulster’s crunch match against Tigers before producing a fine display on his 100th appearance in the 14-13 win.
Two-time British & Irish Lions tourist Rory Best was also in action for Ulster as they edged out the Premiership side to secure their quarter-final berth in the Champions Cup.
There were also plenty of Lions in action for Munster as they joined Ulster in the Champions Cup knockout stages with a bruising victory against Exeter Chiefs at Thomond Park.
Peter O’Mahony and Conor Murray, the latter making his 50th appearance for the club, were both influential as the Irish province came from behind to beat Chiefs 9-7.
O’Mahony, who captained Lions in their 30-15 first Test defeat to New Zealand at Eden Park in 2017, was at his destructive best against the Premiership side alongside fellow Lion CJ Stander as Munster secured their place in the quarter-finals.