Le Maistre, the grandson of former St Martin Connetable John Germain, netted just four minutes from the end of extra time to book Grouville a first-round match away to Trinity on Saturday afternoon.
St Martin had the better of the early exchanges and they went ahead early on with a near-post glancing header from Robbie Searle.
Exchanges were fairly even up to half time before Grouville equalised when a St Martin defender put the ball into his own goal.
John Searle restored St Martin’s advantage for the second time when he netted from close range following a melee in a six-yard area.
Veteran Martyn Noel pounced to make it 2-2 after Peter Gough’s header following a corner kick came back into play off a crossbar.
St Martin goalkeeper Paul Aubert pulled off a couple of good saves in extra-time before finally being beaten by Le Maistre with a penalty shoot-out looming.