Mapletoft, moved from his usual fly-half spot to centre in a switch with Mike Catt, landed six penalties out of six as he followed up his 17-point haul in last weekend's European Challenge Cup win over French side Auch.
He also converted after setting up centre partner Scott Staniforth's 27th-minute try.
When Worcester fly-half Tommy Hayes spilled the ball inside his own 22, opportunist Mapletoft kicked ahead to enable Staniforth to pounce for the touchdown.
Worcester, seeking a second victory of the season to try to increase the gap above bottom club Harlequins, their only victims so far, briefly raised their hopes at the start of the second half when Catt lost possession in front of his own posts.
Hayes snatched up the loose ball and Worcester quickly moved it through centre Dale Rasmussen and replacement Ben Daly for flanker Pat Sanderson to dive over in the right-hand corner.
But Hayes who had landed one of his two first-half penalty kicks, missed the conversion and Mapletoft added three more second-half penalties to make sure his side took the rewards for a solid all-round display in which their forwards dominated.
Hayes was fortunate to escape a yellow card just before half time.
He had already escaped when a questionable challenge felled Michael Horak after the full-back kicked ahead.
But there was no doubt when he dangerously took out Delon Armitage as the young winger jumped to collect a high kick.
Referee Chris White awarded the penalty and admonished four players involved in the ensuing dust-up but allowed Hayes to escape scot-free.
Worcester never gave up trying for a losing bonus point and were denied a try when left-wing Ben Hinshelwood was just forced into touch after appearing on the right flank with a powerful run.
They forced Irish back throughout the eight minutes of injury time and were finally rewarded with a penalty try after the home side had been penalised three times in front of their own posts.
Replacement Ben Gollings added the conversion but it was not enough to give Warriors a tangible reward for a gutsy performance marred by losing the ball on too many occasions.
For London Irish:Try: StaniforthCon: MapletoftPens: Mapletoft 6
Worcester: Tries: Sanderson, penaltyCon: HayesPen: Hayes