Leinster coughed up a 19-10 half time lead as the Llanelli Scarlets maintained their 100% record at Wrexham's Racecourse Ground, outscoring the visitors three tries to one in a 24-22 win.
Former England A prop Ricky Nebbett grabbed his first Leinster try in his eighth appearance, and even though Kiwi fly-half David Holwell kicked 17 points, it was still not enough for Declan Kidney's side.
Leinster's third straight loss in the Principality had Holwell frustrated. "It was there for the taking, but at the end of the day we tried to play too much rugby when we should have held it and kicked more," admitted the man-of-the-match.
"They played good football and scored some fine tries. They took their chances, so all credit to them."
Holwell took a firm hold on the game after sloppy Leinster defence had seen Matthew Watkins in for the game's first try. He slotted over four penalties to reassume the lead, and with Scarlets' flanker Gavin Thomas sin-binned, Nebbett was mauled over on 26 minutes after winger John McWeeney had set up a close-range lineout.
Leinster lost loosehead John Lyne for a ruck offence before the break, and paid dearly for a slack second half.
Aisea Havili scored a try after picking up a loose midfield pass from Holwell, and although the New Zealander made amends with his fifth penalty on 66 minutes, Barry Davies' left wing effort proved crucial with Gareth Bowen adding the telling conversion.
Scarlets: Tries: Matthew Watkins, Aisea Havili, Barry Davies; Cons: Gareth Bowen; Pen: BowenLeinster: Try: Ricky Nebbett; Con: David Holwell; Pens: Holwell 5
15 - D Hewitt14 - J McWeeney
E Byrne for J Lyne (48 Mins);N Ronan for S Jennings (63 mins)