A pulsating match finished four-all after 90 minutes and five-all after extra time.
St Lawrence went a goal down inside two minutes but by half time they led 4-1 after a brace from Ross Crick and singles by Carl McConnell and Rupert Murray.
But St Brelade hit back after the resumption with Island striker Joe Murphy going on to claim a hat-trick and Gary Freeman a single.
Murphy's final goal came deep into time added on for stoppages but 15 minutes later St Lawrence went 5-4 ahead through substitute Kyle Rea.
The Shield holders then had Aaron Parkinson and Tommy Wigley sent off late in extra time, both following second cautions, and a minute into time added on again, Jordan McBride equalised for St Brelade with a left-foot volley into a top corner.
McBride's strike sent the tie into a penalty shoot-out with St Brelade winning 4-3.