Pro 12 side Scarlets and Championship club Jersey shared 11 tries as the Welsh outfit ran out 40-27 winners.
The islanders went ahead through Tom Howe's try but scores from Tom Price, James Davies and Aled Davies saw the visitors lead 19-5 at the break.
Will Boyde, Rhodri Williams and teenager Ioan Nicholas touched down for Scarlets after half time.
But tries from James Freeman, Dave Markham, Nick Selway and a penalty try gave Jersey supporters plenty to cheer.
"It was a good hit out for us and it's been a great occasion I think," said Scarlets head coach Wayne Pivac.
"We certainly have plenty to work on, a few weaknesses were shown up today in particular the line-out drive.
"We want to improve on last year and we want to get further up the table and break into that top four."
Scarlets, who were missing a number of World Cup players, found themselves 5-0 down midway through the first half when winger Howe went over in the corner after a lively break from centre Fautua Otto.
That seemed to stir the visitors into life though, with debutant Price crashing over beneath the posts after a good run from centre Regan King sent him clear.
Scores from flanker James Davies and then scrum-half Aled Davies before half-time meant it would be a long way back for the Channel Islanders.
Replacement Boyd scored moments after coming on, in a scrappy start to the second half that was to be dominated by the impressive Jersey driving maul.
Freeman, Markham and Selway all crashed over from powerful drives, with Scarlets replacements Williams and 17-year-old Nicholas grabbing tries at the other end.
A late penalty-try after more pressure from the Jersey pack ensured the hosts won the second-half 22-21, with Lewis Robling's conversion with the final kick of the game being their only successful kick at the posts all afternoon.
"Overall I'm pretty happy with a decent performance," the islanders coach Harvey Biljon told BBC Radio Jersey.
"We missed 11 points off the floor which could've changed the score line a bit and probably put both teams under pressure.
"We've been working hard as a group and I think the forwards can take a lot of credit. A lot of what we're going to do this season is going to come down to the platform the forwards give."
Jersey's final pre-season game ahead of the new Championship campaign is next Saturday against Blackheath, while Scarlets take on Newport Gwent Dragons at Rodney Parade on Friday.