Tigers fullback Andy Goode proved to be the difference between the two sides racking up a personal tally of 17 points for the visitors.
The victory was the first for Leicester in the Premiership since December 2003, and the first since the sacking of coach Dean Richards and it would have gone a long way in erasing the bitter memories of a dismal season, thus far, for the powerhouse club.
Exiles opened the scoring with a Mark Mapletoft penalty, but Leicester answered back almost immediately as Goode put the first of his four penalties over. Mapletoft added another to restore the lead, but the Tigers soon began taking control.
Centre Daryl Gibson made the breakthrough when he combined with fly-half Jaco van der Westhuyzen to put Freddie Tuilagi close to the line before retrieving the ball and forcing his way over in the corner. Goode was on hand to slot the conversion.
The Tigers never looked back going 19-6 ahead 10 minutes after the break before losing Rowntree and then Smith to the sin bin. Irish, however, were unable to take advantage of the extra man – picking up just three points via Mapletoft's third penalty goal.
Leicester kept in front and a drop-goal by Goode in the 63rd minute proved to be enough for the win.
For London Irish:Pens: Mapletoft 3
For Leicester:Try: GibsonCon: GoodePens: Goode 4DG: Goode