Simon Easterby’s men dominated the opening exchanges and raced into a 13-point lead thanks to a Rob McCusker try and the boot of Rhys Priestland.
But Leinster fought back with touchdowns from Martin Moore, Jordi Murphy, Darren Hudson, Dave Kearney and 22-points from the magnificent Jimmy Gopperth.
The frustration of last year’s play-off exit was clearly evident as the Scarlets came flying out of the blocks.
They made a commanding start to the clash with a thunderous Aaron Shingler burst laying the foundations for younger brother Steven to open the scoring.
They quickly doubled their lead thanks to the boot of their new centre but it could have been more after his midfield partner Scott Williams was flattened by a high-tackle with the try-line at his mercy.
Another rampaging break by the Wales back-row allowed Williams to send captain McCusker crashing over beneath the posts.
Shingler converted but the Scarlets soon conceded a soft try as Leinster comfortable rumbled over from a five-metre lineout.
Gopperth added the conversion and a penalty shortly after the half-hour mark to cut the Scarlets’ lead to just three-points.
Shingler hit-back with a penalty to restore the hosts’ six-point advantage at the break after Leinster drifted offside in midfield.
But Leinster began the second-half in perfect fashion, producing a try inside two minutes of the restart.
The ball was spread wide late call-up Brendan Macken bumped off Gareth Owen and offloaded to Murphy who bundled over in the corner.
Gopperth was on target once again and he quickly added a penalty to stretch the lead to four-points.
Shingler hit-back after Eoin Reddan was penalised for a crooked feed at the scrum. But Gopperth responded as Leinster kept their noses in front before they ran riot in the closing stages.
A Jack McGrath break down the left released Hudson to scorch home in the 68th minute to seal the win.
They continued to dominate and put the result well beyond doubt as Gopperth and Kearney crossed the whitewash.
