Paddy Jackson, edged off the Ireland bench by utility back Ian Madigan for Saturday's clash with France, responded with consummate professionalism.
The young out-half scored two tries and made another for Tommy Bowe in a virtuoso performance.
In his first appearance in the white jersey for 16 months, crowd favourite Stephen Ferris looked strong and committed as he came on after 60 minutes to a rapturous Ravenhill ovation.
Ulster could have had five points on the scoreboard within the opening 90 seconds, Jared Payne devouring half the length of the pitch before supplying Bowe - only to see the Monaghan man bundled into touch a few metres from the line.
Nick Williams - back in contention and sporting a new Mohican-style haircut - was heavily involved in the opening exchanges, but with three forward plays ruled against him in the space of five minutes, he suffered a frustrating start to his return to first team duty.
Jackson showed great opportunism in the 16th minute, following up on his own low prodded kick and capitalising on the unpredictable bounce to pick up 10 metres from the line and ground the first try which Ruan Pienaar converted.
Jackson then unselfishly fed Bowe five metres from the line only two minutes later, presenting the winger with a short run between the posts for one of the simplest tries he will ever score in an Ulster shirt.
Pienaar converted once more and after Stuart McCloskey had come close to marking his first full cap with a try, Rory Scholes similarly impressed at the opposite end of the pitch with a last-ditch tackle on Kristian Phillips in the corner.
With Callum Black exiting on a stretcher just before the half hour, Tom Court took over at loosehead prop.
Although Olly Barkley opened the Scarlets' account with a well-taken 33rd minute penalty, Ulster continued to dictate the play with the Jackson-Bowe tandem once again instrumental in the third try.
This time Bowe played the role of architect, cutting the Scarlets defence to ribbons with a diagonal run before supplying the out-half on the 22 with a neat flicked pass. Jackson raced over the line, his score once again converted by his half-back partner Pienaar.
The Scarlets' riposte was rapid, scrum half Gareth Davies grabbing a try after just outpacing Pienaar to dive onto a well-directed kick four minutes before half-time.
Barkley converted for a 21-10 scoreline but then missed his chance to bring his side to within eight points of Ulster with an errant penalty attempt.
Jackson continued to excel in every aspect of his game as the second half got underway, astutely charging down a Scarlets kick and setting the wheels in motion for Ulster's next attack.
Two narrow penalty misses from the boot of Pienaar followed, both from the Scarlets 10-metre line, but Ulster continued to plough forward undeterred in their quest to close out the result.
Perhaps the biggest roar of the evening came on the hour as fit-again back rower Ferris made his entry just after a successful penalty by Barkley, replacing Williams at number 8.
Ferris, who was joined on the pitch by fellow replacement Craig Gilroy, wasted no time in showing he had lost none of his characteristic tenacity, dragging an opponent 10 metres backwards with his very first tackle.
With the Scarlets still perilously close on the scoreboard as the final 10 minutes loomed, a pinpoint cross-field kick from Jackson picked out Scholes before he and Darren Cave somehow contrived to get in each other's way, and the chance was lost with another knock-on.
Ulster kept possession well in the closing minutes, coming up against some stubborn Scarlets defence as they protected their 22 with conviction.
Pienaar succumbed to what appeared to be a shoulder injury three minutes from time, replaced by Michael Heaney, but Ulster persisted until the final play and were rewarded right at the death.
Prop Court piled through a mass of bodies to stretch just over the line and television match official Marshall Kilgore confirmed the score.
Injury worries to Pienaar and Black aside, the result proved to be a good night's work for Ulster who are now just three points behind Munster - lying in third place in the PRO12 table.