Leinster ground out a second win of the Pro12 season in a drab encounter at Parc y Scarlets.
The visitors led 6-3 at the break thanks to Isa Nacewa kicking two penalties to Dan Newton's single shot.
Scarlets wing Lee Williams went over from the game's best move seven minutes after the break and Newton's conversion put them into the lead.
But Nacewa kicked two more penalties while Rob McCusker was in the sin-bin and Leinser held on.
The Scarlets marketed the game as the 5,000th in the combined history of Llanelli RFC and the region it spawned in 2003.
But the contest did not live up to the traditions of the west Wales entity or the reptutation of their visitors from Dublin.
The game was dominated by defence, peppered with mistakes on both sides and - apart from wing Williams - only rival kickers Newton and Nacewa were able to profit.
Nacewa came out on top, kicking five penalties to Newton's solo effort and conversion, the home fly-half also missing with one attempt at goal.
Leinster skipper Nacewa put his side 9-3 ahead two minutes after the break with a third penalty.
But the Scarlets produced the game's best moment to conjure a try more in keeping with the occasion.
Scrum-half Riki Tipuna stole away from a ruck and athletic lock Aaron Shingler's well-timed pass ensured diminutive 14 Williams a free run to the line.
Newton converted to put his side into the lead, but it was short-lived.
Flanker Rob McCusker acted illegally at a ruck and watched Nacewa's fourth penalty bisect the posts from the discomfort of the sin-bin.
Nacewa added a fifth penalty while McCusker sat out his punishment.
After the flanker returned, the hosts built some momentum, but Newton was unable to punish lock Devin Toner's offside efforts at a ruck.
Try as the might, the hosts were unable to make further headway into Leinster's resolute defence.
Scarlets: Ford; Lee Williams, Reynolds, Maule, Fenby; Newton, Tipuna; I Thomas, Phillips, R Thomas, Shingler, Day, McCusker, J Edwards, Morgan.
Replacements: Liam Williams for Ford (52), A Thomas for Newton (72), John for I Thomas (62), R Jones for R Thomas (68), Welch for Day (27), Pugh for J Edwards (71).
Not Used: Hawkins, G. Davies.
Leinster: Nacewa; Carr, O'Malley, Fitzgerald, D Kearney; Madigan, Willis; van der Merwe, Strauss, White, Browne, Toner, R Ruddock, Murphy, Auva'a.
Replacements: Hagan for White (40).
Not Used: Harris-Wright, McGrath, Flanagan, C. Ruddock, Cooney, Macken, Hudson.
Ref: John Lacey (Ireland).
Assistant Referees: Neil Hennessy, Hugh Watkins (both WRU)
Citing Commissioner: John Charles (WRU)
TMO: Tony Rowlands (WRU)