Llandovery lifted the Konica Minolta Cup for the first time in the club's history thanks to a dramatic last-gasp kick from fly-half Howard Thomas.
The village side from West Wales produced a sensational try seven minutes into injury time after Craig Evans looked to have stolen the cup for Cardiff.
Drovers replacement hooker Endaf Howells thundered over to level the scores 18-18 before 21-year-old Thomas kept his cool to guide the conversion over the posts and spark wild celebrations at the Millennium Stadium.
Yet again the cup final threw up a late twist but it was the least Llandovery deserved for scoring the only tries of the game.
Favourites Cardiff thought they had their eighth cup title in the bag when outside half Evans put them ahead for the first time in the game just a minute from time and then added a second drop goal in injury time.
Yet the Drovers have revelled in the Welsh Cup this season and summoned one final effort through Gareth Williams and Phil John that saw Howells touch down.
Few people gave Llandovery any chance of lifting the cup, despite knocking out Cup Kings Llanelli in the semi-final. Cardiff may have been playing next door to their Arms Park ground but they were never allowed to settle as Llandovery took a shock lead inside three minutes.
Centre Richard Jones coughed up the ball under the weight of a crunching tackle and that left the back door open with no one at home. Llandovery scrum half Rob Walters booted ahead for wing Jenkins to grab the most memorable score of his life.
Llandovery led 10-6 at the break but Cardiff's power up front allowed Evans to keep the scoreboard ticking over until the 86th minute. However, the west Wales side pulled one last huge effort out of the hat and were rewarded with a piece of history that few of their vocal supporters will forget.
Cardiff: Pens: C Evans (4). Drop Goal: C Evans (2)
Llandovery: Tries: V Jenkins, E Howells. Cons: H Thomas (2). Pens: H Thomas (2)
Cardiff: J Roberts; E Jones, R Davies, R Jones, L Andrews; C Evans, T Isaacs; R Gill, R Johnston, S Roberts, C Stamatakis, J Down, G Lucas, A Powell, G Gravell (Capt)
Replacements: R Price, G Gunter, A Murphy, M Amos, N Hampson, D Allinson, L Halfpenny
Llandovery: I Davies; O Rowlands, M Jones, J Lewis, V Jenkins; H Thomas, R Walters; P John, E Phillips, A B Jones, T Walker, A Davies (Capt), E Gwynne, J Mills, G Williams
Replacements: E Howells, AR Jones, S Covington, G Bennett, A Williams, G Jones, E Evans
Referee: Hugh Watkins (Maesteg)