Mapletoft struck the winning penalty in the 66th minute after the home team's Tim Stimpson had put them ahead by 19-17 with a penalty in the 59th minute, this after the visitors had enjoyed a 17-13 half-time lead.
The experienced Mapletoft, preferred to Barry Everitt this week in the injury-enforced absence of player-coach Brendan Venter, missed the conversion attempt for prop Mike Worsley's try, but his five penalties were absolutely crucial
Worsley dotted down in easy circumstances on the stroke of half-time after a strong run from wing Paul Sackey saw the ball moved wide. Worsley's try was in response to a drop-goal from Ireland's Geordan Murphy and a try from Leicester's John Holtby shortly before half-time after a try from fullback Stimpson in the second minute of the game.
This win for London Irish gives them a season 'double' over Leicester, having won their previous Zurich Premiership fixture by 27-7 in November last year, but most importantly it may just help them beat off relegation fears, with Bristol and Bath due to meet in a crunch fixture on Sunday.
Exiles are now in ninth place on the Zurich Premiership table with 35 points from 21 games, while Bristol and Bath are in positions 11 and 12 respectively, both with 32 points from their 20 matches.
For Leicester, this loss – their ninth of the season – has ruled out any remaining hope of qualifying for the Zurich Premiership Final later this month.
For Leicester Tigers:Tries: Stimpson, HoltbyPens: Stimpson 2DG: Murphy
For London Irish:Try: WorsleyPens: Mapletoft 5