Priestland's fly-half rival, Ceri Sweeney, scored all of Cardiff's points - a try, a conversion and a penalty - as they took a 10-9 lead into the break.
But Priestland's 62nd-minute try showed the way for the Scarlets, with replacement Stephen Jones, a steadying presence in the final quarter, adding the conversion.
The Scarlets were forced into a number of late changes in personnel, the most notable seeing former All Black Regan King replace Jonathan Davies in midfield and with Johnathan Edwards' wife expecting their first child, Rob McCusker moved to the blindside flank and Ben Morgan, who has impressed for Llanelli this season, came in for his first start at number 8.
Cardiff lined out as selected, with new Wales squad member Scott Andrews getting the nod at tighthead prop. Andries Pretorius made his first start of the campaign at blindside flanker, while Michael Paterson and Chris Czekaj started in the second row and at full-back respectively - the latter taking over from the injured Ben Blair (knee).
The Scarlets had the better of general play in the opening half, but could not cross the hosts' whitewash. The closest they came was when Iestyn Thomas scrambled over the line on his back, but television match official Gareth Simmonds ruled that the prop had failed to ground the ball.
Presented with a tough kick from close to the right touchline, Sweeney missed an early penalty opportunity before Priestland opened the scoring in the 10th minute, punishing Sam Warburton for coming in at the side of a ruck.
Priestland missed a second kickable chance, and the Blues forwards took their cue to raise the tempo. Team captain Xavier Rush made some good yardage, and Sweeney popped over the levelling penalty on the quarter-hour mark.
Blues winger Leigh Halfpenny was off target with a long range penalty attempt, before Cardiff were given a numerical advantage. Scarlets winger Morgan Stoddart was sin-binned for going offside, the twelfth penalty of a stop-start opening quarter.
Before the penalty was awarded, a dangerous tackle by Stoddart on Tom James left the Cardiff winger requiring treatment for a dead leg, and he was unable to continue.
From the resulting penalty, Cardiff moved menacingly forward under the baton of Kiwi centre Casey Laulala and Rees popped a lovely pass, while drawing two defenders, for his half-back partner Sweeney to run in unopposed for the game's opening try. Sweeney landed the simple conversion for a 10-3 scoreline.
James' replacement Richard Mustoe was penalised for going offside, allowing Priestland to double his and the Scarlets' tally, but the Scarlets number 10 was then wide with a kick from inside his own half. It came after Blues flanker Pretorius was sin-binned for going off his feet at a ruck.
The Scarlets kept pressing for scores as the first half came to a close. They came close to a try after scrum half Tavis Knoyle sparked a quick counter after a loose kick from Rees.
Stoddart broke forward and kicked ahead, but the busy Rees redeemed himself with a try-saving tackle on the Scarlets number 14. Thomas' subsequent attempt at scoring was ruled out by the TMO, as he lost the ball forward.
Priestland's third successful penalty, after a high tackle by Laulala on Stoddart, made it a one-point game and the Scarlets missed two further scoring chances as they ended the half on the attack.
Rees was sin-binned for a deliberate knock on, however Priestland missed the resulting penalty kick and newcomer Morgan, with the line in his sights, lost the ball forward in the final play, under pressure from Pretorius.
The 14-man Blues were living on the edge and the Scarlets failed to convert another chance when King's pass for the supporting Stoddart was ruled forward.
In an attritional second half, the Blues' replacements bench was quickly emptied and with Mustoe forced off with a blood injury, scrum half Lloyd Williams was briefly sent onto the wing.
Paul Tito replaced fellow New Zealander Paterson for the Blues, and he was involved in a promising attack which Stoddart ended with a interception. The Scarlets' solid scrum was added to by the introduction of Wales and Lions hooker Matthew Rees, the region's captain this season.
Just when it looked like they might have to rely on place-kicks, the Scarlets opened up the hosts' defence for that crucial try. Stephen Jones, who had just replaced Dan Evans, used scrum ball to put in a deft cross-field kick, Lamont rose highest to tap it back for the supporting Priestland to gather and romp over from 40 metres out.
Jones tagged on the conversion for a six-point lead for the men in red, which Priestland could have added to - he missed a long range penalty attempt in the 71st minute.
But Cardiff lacked the control to engineer a response and although Scarlets replacement lock Dominic Day was sin-binned for going offside in the dying minutes, Nigel Davies' side held on for a character-building victory.
The result marked Cardiff's first home loss since Gloucester won at Cardiff City Stadium in the LV= Cup last March, and only their second defeat in nine Welsh derby matches.
|Cardiff Blues Score Card|
|Scarlets Score Card|