Groombridge has scored in every round of the competition and his stunning first-half goal was key as Deal made it back-to-back league and cup doubles.
“Charlie has scored in every round he’s played in this tournament now,” said Deal manager Andrew Crush.
“He’s a big game player and I thought he was class, he ran his socks off, won his headers and his individual battle.
“It was a really good performance from the boys, we had quite a few young ones out there and we’ve come out on top.
“It was a really tough game. We’ve had some matches earlier in this competition where we’ve managed to rotate players and stuff but they gave us a proper game.
“It’s back-to-back doubles now. Last season we got a bit of fortune with other teams playing ineligible players but this year we wanted to win it the proper way
“We’re top scorers in this competition again so we’re always going to score goals.”
Deal striker Jamie Kennedy had already gone close by the time his low shot was well saved by keeper Hadley Glock. But Deal kept the ball alive and Kennedy unselfishly crossed for Josh Byford to have a 17th-minute tap-in to put his side ahead.
Sheppey responded and skipper Aiden Clark shot narrowly wide from the edge of the box before the Ites equalised on 31 minutes as good pace by Oladipo Durowoju saw him cross for Finlay Gooding to hook home.
Striker Davey Johnson did well to hold off the attentions of Deal defender Wes Maggs but his 20-yard right-foot shot was well saved by keeper Sam Hignett.
Sheppey’s Gooding then shot over and Hignett did well to deny Durowoju before Deal regained their lead in style.
Good work by Sam Carpenter on the right saw him set up Groombridge, whose crisp right-foot shot arrowed into the top corner from 25 yards.
Kennedy had three chances in the second half to add a third but was unable to find a way past Glock.
The key moment arrived with 12 minutes left when Deal keeper Hignett produced an incredible one-handed save to deny Durowoju an equaliser as his 25-yard right-foot shot seemed destined for the top corner.
Deal promptly killed the tie off with four minutes left when Byford did well to keep the ball alive inside the area and his cross eventually fell to Kennedy, who turned the ball in from close range.
Sheppey: Hadley Glock, George Miles, Harrison McCall, Aiden Clark, Louis Way, Eduard Mare, Oladipo Durowoju, Bailey Catherick, Davey Johnson, Teddy McIntyre, Finlay Gooding, Kenzie Monahan, Liam Parle, Miko Tkachuk, Alex Barnes, Harry Rogers.
Deal: Sam Hignett, Wes Maggs, Matthew Chambers, James Rigby, Charlie Groombridge, Zak Hammond, Joe Roberts, Jamie Kennedy, Harry Andrews, Sam Carpenter, Josh Byford, Douglas Saunderson, Jude Williams, Kyle Davidson, Owen Wilkinson, Alfie Wilkins.
Referee: Ismail Isik.
