14 points from Ian Madigan consigned the Scarlets to their first defeat of the season in a tight match at the RDS in Dublin.
Tries from Hadleigh Parkes and Gareth Owen plus the boot of Dan Jones had given the Scarlets a deserved lead at the break.
However the visitors were kept scoreless in the second-half as Leinster edged out a hard-fought victory.
The main positives for Wayne Pivac will be the way his players performed in the first-half and how their strong defence in the second-half secured a losing bonus-point.
The Scarlets started strongly and were rewarded when Parkes crashed over under the posts following pressure from the visitors’ pack.
However Leinster worked their way back into the match and an incredible driving maul resulted in Jake Ball being sent to the sin-bin.
The home side looked dangerous in the Scarlets 22 and the visitors would consider themselves fortunate to only concede three points to the boot of Ian Madigan.
Another penalty from Madigan left the home side just a point behind but it was not long before the away team extended their lead.
Following good work from the Scarlets backline, a Gareth Davies snipe was supported by John Barclay who put Owen in the corner with a brilliant pass to score.
Madigan and Dan Jones exchanged penalties as the Scarlets went into the break deservedly leading 15-9.
The home side came out of the half-time break determined to make amends and following numerous pick-and-goes, Mike Ross mananged to drive over.
Leinster continued to put pressure on the visitors as the Scarlets began to get on the wrong side of referee Leighton Hodges.
Another Madigan penalty extended the lead to four points as the match entered the final quarter.
While the Scarlets enjoyed 55% possession in the first-half, they could only muster 25% of the second-half possession with 15 minutes remaining.
And with ten minutes left, Johnny Sexton appeared to have scored the try which would have secured victory following a neat wrap-around.
However, the score was ruled out for crossing in midfield.
With their perfect league record on the line, the Scarlets built pressure as they searched for their first win at the RDS since 2007.
And just when it looked like the visitors were in a position to score, loose hands by lock Maselino Paulino gifted Leinster the ball and the victory.
