Ystrad Rhondda were defending a two year unbeaten home record stretching back to defeat against Bridgend.
Llandovery opened the scoring following fine interplay by the backs and an incisive run from Gareth Davies, who put Jeremy Griffiths over for an unconverted try. The visitors went further ahead thanks to some effective driving from their forwards and Adrian Davies crossed.
Ystrad defended stoutly and countered dangerously, and made it 3-10 with a penalty.
Llandovery finished the home side off after the forwards set up a Nick Griffiths break, which allowed Simon Davies to cross. Griffiths converted to complete a hard fought win.