The Scarlet opened their EDF Energy Cup campaign with an impressive 26-17 victory over Saracens at Vicarage Road on Sunday, with the boot of Wales pivot Stephen Jones proving to be the difference between the two sides.
Faced with a third match in the space of nine days, Saracens boss Eddie Jones made 14 changes from the side which won at Bristol in the Guinness Premiership on Wednesday night.
Young flanker Andy Saull was the only survivor in the XV, but the Welsh side, fielding their strongest available side, did not have things all their own way before securing victory when they drove Kees Meeuws over for what proved the decisive score in the 48th minute.
The other Scarlets try came from number eight David Lyons and Stephen Jones contributed 16 points with four penalties and two conversions.
Saracens replied with tries from centre Francisco Leonelli and England Under-20 left-wing Noah Cato, both converted by Scottish international fly-half Gordon Ross who also kicked a penalty.
Scarlets, seeking a confidence-booster ahead of their opening Heineken Cup match against Harlequins on Saturday, were far from convincing after conceding a total of 66 points in defeats by Edinburgh and Glasgow in their last two Magners League matches.
They looked far from secure in defence as young Cato provided all the trouble they could handle.
Saracens did not get off to the best of starts, with Ross missing a penalty attempt after only 25 seconds.
They fell 7-0 behind when Scarlets, spurning the chance of an easy penalty, opted for the scrum and drove the Sarries pack back over their own line before Lyons dropped on the ball for their opening try, converted by Jones.
Saracens, prompted by scrum-half Moses Rauluni, leading the side for the first time, hit back to take the lead with two tries in a six-minute spell.
Leonelli took the final pass from Cato to cross in the 12th minute and then Cato almost scored four minutes later when, chasing his own deep kick, he won the chase with Nathan Brew but just failed to reach the ball before it rolled dead.
It took him only two minutes to make amends, racing in to finish off a move instigated by Ross and centre Alex Goode.
Ross converted both to put the home side 14-7 ahead but they paid for a string of infringements as three penalties from Jones wiped out the lead to send Scarlets in with a 16-14 half-time advantage.
It could have been more but Scarlets scrum-half Martin Roberts squandered a great opportunity when he knocked on and then, after snatching the ball from the base of a ruck and racing down the left wing, slipped as he tried to cut back inside Saracens' full-back Dan Scarbrough.
The lead lasted just three minutes into the second half before Ross boosted Saracens back in front with a penalty from a metre inside his own half.
Scarlets regained the lead when they powered Meeuws, returning to the side from injury, over in the 48th minute, with Jones adding the conversion from the left touchline and the fly-half stretched their lead to nine points with his fourth penalty after 63rd minutes.
The sin-binning of Scarbrough reduced Saracens to 14 men in the 66th minute but the numbers were levelled when Roberts soon followed him.
Owen Farrell became the youngest English player in the professional era when he replaced Goode in the 67th minute.
At 17 years and 11 days, the son of Andy Farrell - the currently injured Saracens centre and former Great Britain rugby league captain - beat the record of Danny Cipriani by 36 days. Cipriani was 17 years and 47 days old when he first played for Wasps at Bristol in the Powergen Cup in December 2004.
Saracens, utilising their bench, mounted a late rally to try and rescue the match but were unable to find a way through.
Tries: Leonelli, Cato
Cons: Ross 2
For the Scarlets:
Tries: Lyons, Meeuws
Cons: S Jones 2
Pens: S Jones 4
Saracens: 15 Dan Scarbrough, 14 Edd Thrower, 13 Francisco Leonelli, 12 Alex Goode, 11 Noah Cato, 10 Gordon Ross, 9 Moses Rauluni (c), 8 Ben Skirving, 7 Andy Saull, 6 Don Barrell, 5 Kris Chesney, 4 Harry Spencer, 3 Tom Mercey, 2 Matt Cairns, 1 Nick Lloyd.
Replacements: 16 Alex Walker, 17 Cobus Visagie, 18 Hayden Smith, 19 Jackson Wray, 20 Mickey Wilson, 21 Owen Farrell, 22 Rodd Penney.
Scarlets: 15 Ceiron Thomas, 14 Mark Jones, 13 Regan King, 12 Gavin Evans, 11 Nathan Brew, 10 Stephen Jones, 9 Martin Roberts, 8 David Lyons, 7 Gavin Thomas, 6 Simon Easterby (c), 5 Simon Maling, 4 Vernon Cooper, 3 Kees Meeuws, 2 Matthew Rees, 1 Iestyn Thomas.
Replacements: 16 Phil John, 17 Ken Owens, 18 Scott MacLeod, 19 Nathan Thomas, 20 Sililo Martens, 21 Rob Higgitt, 22 Rhys Priestland.
Referee: James Jones (Wales)
Touch judges: Phil Connett (Wales), Mike Bethell (Wales)
Television match official: Roy Marfleet (England)