Fellow winger Andy Fenby went over twice and the impressive Liam Williams added another - as Wales fly-half Rhys Priestland came off the bench to make his 100th appearance for the region.
The win - in Simon Easterby's first competitive game as head coach - was the Welsh side's first over Leinster since 2009, but came against a squad hit heavily by injury and missing eleven internationals.
In Jonathan Sexton's absence, 23-year-old Ian Madigan kicked ten points for the Irish province and added a consolation try, but his side were overpowered by a focused Scarlets team that repeatedly turned down kickable penalties en route to a seven-try haul.
Elsewhere Cardiff Blues made it two from two in the day's Ireland vs Wales match-ups, edging out Connacht in a hard-fought 13-9 win.
Phil Davies marked his first game as the Blues' director of rugby with a win, behind the goal-kicking of experienced fly-half Ceri Sweeney.
Scrum-half Lewis Jones, 19, scored his first try for the club as the Welsh side took advantage of a strong wind in the first half to go in 13-0 up at the break.
With ex-Cardiff fly-half Dan Parks unavailable to make his Connacht debut, Miah Nikora took the kicking duties and, after struggling early on, hit three second-half penalties to restore some pride.
It was Munster who had the last laugh in the final game of the night, beating Heineken Cup semi-finalists Edinburgh 23-18 at Murrayfield.
Wing Tim Visser continued in the electric form he showed on tour with Scotland this summer, running in a hat-trick for the hosts, but it was not enough to overcome a confident Munster performance led by a 18-point haul from fly-half Ian Keatley.