In truth the home side dominated the clash but some wasteful kicking from Dragons centre Lewis Robling - who replaced Jason Tovey on the kicking duties when he picked up a first-half injury - was the difference between the sides.
Robling missed four penalties as a try from Johnathan Edwards, converted by Dan Newton, set the stage for Thomas to remind his former employers of his skills.
The home side should have taken the lead on 11 minutes but the usually reliable left boot of No. 10 Tovey let him down.
Minutes later Dragons full-back Newton was then short with a penalty effort from distance as the scores remained level.
The deadlock was broken in the 18th minute when a flowing move, involving both props, saw Newton give the decisive pass to Edwards and the flanker gleefully went over.
Newton added the extras to make it 7-0 but the Dragons got to within four points when Tovey knocked over a penalty on 24 minutes.
Nine minutes later the Scarlets wasted an opportunity to stretch their lead when Newton snatched at a long-range penalty effort and the visitors should have been made to pay with the half-time whistle beckoning.
First Robling - with Tovey struggling with a knee injury - missed a sitter of a penalty before Toby Faletau's wayward pass to Tonderai Chavhanga spoiled what looked like a near-certain score in the corner.
However despite their best efforts the home side did end the half on a high when Tovey kicked a penalty following Kieran Murphy's obstruction.
And it was a double boost for the Rodney Parade faithful with the No. 8 also being sin-binned for the offence.
Some Scarlets nous saw them survive the ten minutes with their one point advantage intact without Murphy and the No.8 and five other fresh faces, including teen wing sensation George North, joined the fray at the 50 minute mark.
Their arrival gave Nigel Davies' side a chance to make it 10-6 in the form of a penalty but a bad night for the place-kickers of both sides continued when Newton's effort slid wide.
With Tovey unable to take the field after the break Robling was tasked with kicking duties but he made it three misses from three attempts with two wayward efforts just after the hour mark.
A night to forget continued for Robling continued with ten minutes remaining when he missed the easiest penalty of the evening.
And their wastefulness was exacerbated when former Dragon Thomas stepped up to kick an impressive penalty to settle it and pile more misery on the struggling Dragons who have just two RaboDirect PRO12 wins to their name.
|Newport Gwent Dragons Score Card|
|Scarlets Score Card|