Within 16 minutes, the Gwent region were 11-0 up as New Zealand outside-half James Arlidge punished mistakes to boot two penalties before wing Richard Fussell stormed over for a try near the left corner after a neat move involving centres Marc Stcherbina and Rory Sidey.
Welsh stand-off legend Stephen Jones put the first points on the board for the Scarlets with a 35-metre penalty and then made a tremendous tackle to prevent Fussell reaching the tryline again.
However, two terrible mistakes by the West Walians in the final 10 minutes of the half gave them a mountain to climb after the interval at Newport.
Both decisions went to video referee Tony Rowlands and both were given as new Dragons hooker Tom Willis, an inspiration to his side on the night, pounced on an awful tap-down from lock Lou Reed at a five-metre defending line-out by the Scarlets.
Arlidge converted but, when the Dragons infringed 15 metres from the line, Jones reduced the arrears to 12 points.
However, another mistake on their own line cost the Scarlets dear.
This time, the ball squirted out from a ruck for Dragons number eight Lewis Evans to pinch the ball and - after a lengthy look at the television replay - referee Jones gave the verdict to the Dragons, with Alridge adding the conversion.
Nineteen points was a long way back for the Scarlets but, within the opening plays of the second period, they chipped six points off that deficit via two Jones penalties and only a brilliant Sidey tackle 10 metres from his line prevented a remarkable try from visiting wing Darren Daniel.
The Dragons did not help themselves either just before the hour when former Scarlets wing Tai Selley was sin-binned for persistent infringements and that mistake was heavily punished as replacement scrum-half Martin Roberts squeezed in for a corner try.
And the Scarlets, watched by the whole Welsh national coaching team including boss Warren Gatland, dramatically brought themselves to within one point of the Dragons when full-back Morgan Stoddart stormed downfield to give Nathan Brew a scoring pass which looked suspiciously forward.
It was set for a grand finale and three times the Scarlets missed sitters to win.
Brew dropped a kick-through with the line at his mercy from 10 metres, Jones sliced a simple drop-goal from 15 metres and a long Stoddart pass to the wing failed to find Dafydd James.
But, in the fourth minute of injury-time, a scrum infringement gave Jones a penalty to win it - and he obliged.