NEW out-half Jimmy Gopperth produced an outstanding display as Leinster came from 13-0 down in the opening quarter to ease their way to a bonus point win.
It took Leinster a long time to get on top, but they ran out deserving winners once they got control of proceedings, and five tries to one told its own story as Simon Easterby's men were unable to build on a promising start.
Leinster were forced to make a late change with Fergus McFadden not being risked because of a tight hamstring, with Brendan Macken starting instead and Darren Hudson added to the bench.
The champions were under the kosh from the outset and while they were forced to field a vastly understrength side, they still started with seven full internationals to eight for the home side.
Steven Shingler gave Scarlets an early lead with a penalty after older brother Aaron had made the hard yards.
Dave Kearney was yellow-carded after six minutes for a high tackle on Scott Williams after the Welsh centre seemed poised to score after a good break. Shingler slotted the penalty from the left to make it 6-0.
Gopperth had the distance, but not the accuracy with his first penalty attempt from halfway before Scarlets countered and struck for the first try of this season's competition after 13 minutes.
Aaron Shingler again made the decisive break and after the ball was worked through several hands skipper Rob McCusker got the vote from the TMO after a suspiciously forward pass and an even more dodgy grounding. But the score was given and Shingler's conversion made it 13-0.
Leinster looked set for a drubbing, but they forced their way back into the game with a simple, but very effective line-out drive from five metres, with Devin Toner taking clean possession and a superb shove resulting in tighthead Martin Moore crowning his first start with a try.
Gopperth nailed the conversion from the left and then eight minutes from the break added a penalty from 35 metres to make it 13-10.
Leinster, though, encountered a lot of difficulty in the scrum and Eoin Reddan could have been pinged more than twice for crooked put-ins as the home side managed to get the shove on most scrums.
Scarlets rarely looked like adding to McCusker's try and had to settle for another penalty from Shingler to lead 16-10 at the break.
Leinster, hammered 45-20 in the corresponding fixture on the opening weekend last year in Llanelli, brought on Mike McCarthy for his competitive debut and they made a superb start to the second half.
Brendan Macken evaded tackles to set up Jordi Murphy and he held off the challenge of hooker Ken Owens to score, with Gopperth adding the conversion from the right as they hit the front for the first time at 17-16.
Gopperth, whose handling, positioning and kicking was outstanding, extended the lead with a 47th-minute penalty.
But Leinster were again temporarily reduced to 14 men when Hudson, on for Luke Fitzgerald, was sin-binned for taking out Liam Williams in the air after 48 minutes.
Fitzgerald was forced to retire because of a tight groin but Leinster coach Matt O'Connor does not expect him to sidelined for long.
Shingler reduced the margin to 20-19 on 57 minutes when Reddan was again penalised for a crooked put-in just outside his '22'.
But with Gopperth continuing to dictate matters, Leinster kept Scarlets at bay and increased their lead just past the hour when the new out-half landed a penalty from 37 metres.
Leinster then struck a telling blow 12 minutes from time when they went shortside from 40 metres and loosehead Jack McGrath displayed awesome pace down the left before popping inside for Hudson to score his first try for Leinster.
Gopperth added the points from the left touchline, but five minutes later was unable to convert his own try from the same spot after Toner had created the opening for the man of the match to secure the bonus point on a night when the Matt O'Connor era got off to a flying start.
Full-back Kearney put the icing on the cake, grabbing Leinster's fifth try in the final play.
Scarlets G Owen (E Phillips 68); L Williams, S Williams (A Warren 79), S Shingler, K Phillips (J Williams 62); R Priestland, G Davies (R Williams 65); P John (R Evans 52), K Owens, S Lee (J Adriaanse 68); G Earle, J Snyman (J Ball 64); A Shingler (C Price 68), J Turnbull, R McCusker.
Leinster – D Kearney; D Fanning, B Macken, G D'Arcy (N Reid 73), L Fitzgerald (D Hudson 43); J Gopperth, E Reddan (I Boss 64); J McGrath (J O'Connell 77), S Cronin (A Dundon 68), M Moore (M Ross 36); T Denton (M McCarthy h-t), D Toner; R Ruddock, S Jennings (D Ryan 77), J Murphy.
Ref – J Lacey (IRFU).