The Tigers, without a win in their last three league matches, were awarded a penalty try for collapsed scrums in each half by referee Martin Fox even though Leicester did not look likely to score from either.
While the Irish will debate the first one, the referee was on safer ground with the second because the Exiles offended consistently during 10 minutes of scrummaging on their own line.
Toby Flood converted both and also kicked two penalties, while the Irish replied with two penalties from former Australia full-back James O'Connor and a last minute try from centre Fergus Mulchrone.
London Irish had one player, prop John Yapp, sin binned and missed out on a losing bonus point after missing a conversion and a drop goal at the death.
Leicester were boosted by the return of three of their England stars, Flood and the Youngs brothers, Ben and Tom, while London Irish recalled captain and lock George Skivington, England winger Marland Yarde and fly-half Shane Geraghty.
Both sides were desperate to end poor runs, especially Irish who started the day second from bottom and having lost their last four games in all competitions.
Leicester came into the match on the back of two LV= Cup wins but had failed to win any of their last three games in the Premiership and made a terrific start, pinning the visitors in their own half for most of the first quarter.
They took a 7-0 lead but Irish bounced back to reduce the arrears to one point and, with better decision making, might have scored a couple of tries.
Leicester fly-half Toby Flood missed a fourth minute penalty from 40 metres and then turned down four chances for a kick at goal and instead went for a try from driving line-outs.
They failed to score from all four but in the 10th minute referee Martin Fox lost patience with Irish, who had consistently offended, and awarded Leicester a penalty try after the visitors collapsed a scrum near their own line.
It appeared a harsh decision as Leicester did not look likely to score but Flood kicked the conversion to make it 7-0.
The Exiles, however, hit back strongly and Yarde and former Australia full-back O'Connor caused Leicester a hatful of problems.
It took a late diving tackle by Flood to stop Yarde scoring from a counter attack and the winger then made a fabulous outside break before putting away O'Connor who was hauled down in the Tigers 22 by winger Miles Benjamin.
Another chance went begging when centre Eamonn Sheridan ignored Topsy Ojo on his outside after great work by O'Connor and then Yarde was just short of the line after making a lunge from a ruck.
Two penalties by O'Connor put Irish back in the game but he missed with one from long range after Flood had made it 10-6 to the Tigers with his first penalty.
Irish suffered a blow two minutes before half-time, losing hooker David Paice with a broken right arm suffered during an assault on the Leicester line.
Leicester brought on a fourth member of the England team from the autumn internationals, prop Dan Cole, in the second half, and Flood extended the Tigers' lead with a 47th minute penalty.
Yarde then came to the visitors' rescue with a try saving tackle on Leicester replacement Adam Thompstone, who had charged down a clearance from O'Connor.
It appeared as though Thompstone's momentum had taken him over the line but the television match official said he had been held up. Yarde was injured in the process and had to go off.
It led to a series of Leicester scrums on the Irish line. The referee warned Skivington after the fourth scrum went down and two scrums later he sent Yapp to the sin-bin.
After 11 scrums in almost 10 minutes the referee again lost patience with Irish, who were being marched backwards, and awarded the Tigers a second penalty try which Flood converted to make it 20-6.
With less than a minute to go centre Mulchrone scored a fabulous try from a counter attack but Darren Allinson missed the conversion. Geraghty missed a drop goal in stoppage time.
- Sports & Recreation
- London Irish
- Toby Flood