The west Wales outfit edged out Coleg Morgannwg with an 18-16 win at the Millennium Stadium - a game enlivened by a pulsating second half.
The first 45 had been tight as the boot dominated the game. Morgannwg led 9-6 at the break with Jared Evans kicking three penalties to two from Sir Gar's Jack Maynard.
But after the break, things livened up with a score each for either side. Maynard continued his one man show for Sir Gar as he went over though Harri Millard soon got Morgannwg back into it, picking up a loose ball and finishing impressively.
That left Morgannwg with a one point lead with just minutes remaining. Sadly for them though, an indiscretion at the contact area was to haunt them, Jones kicking the resulting penalty to secure the win.
In truth the early stages of the match were a quiet affair, wing Jack Huntley going close for Morgannwg before being tackled into touch.
Maynard went similarly close for Sir Gar but after making a clean break, the young Scarlets fly-half slipped on the Millennium Stadium turf and the momentum was lost. There was little between the teams and few clear cut chances, Maynard and Evans exchanging a penalty apiece to leave the sides locked at 3-3.
In such a tight encounter, the contact area was proving vital with neither pack yielding an inch. It was physical stuff and Morgannwg, who had dominated Llandovery up front last week, were not finding it as easy this time around.
They did though, have the boot of Evans in their armoury and after winning a couple of penalties at the contact area, the fly-half booted his side into the lead. The two scores came after a second effort for Maynard and Sir Gar and at the break, Morgannwg led 9-6.
If the first period had been a battle of the boot, the second was far more entertaining despite no initial change to the score. Sir Gar were seeing more of the ball though the Morgannwg defence were holding firm as the no holds barred hits continued.
But despite their defensive heroics, Morgannwg could only hold out for so long and they eventually yielded as Maynard, who has played Premiership rugby for Carmarthen Quins this season, went over following a sustained period of pressure.
The score was deserved reward for the west Wales outfit after they had put together phase after phase of attack.
It came after Sir Gar decided to run a kickable penalty and when the ball was spread right, Maynard found a whole in the Morgannwg defence to creep over.
The fly half converted and after an hour, his side led 13-9.
Trailing by four going into the last 15 minutes, Morgannwg needed to respond and they did so impressively as they camped themselves inside the Sir Gar half. Aled Summerhill and Tom MacAloon were their primary threats with ball in hand though initially, they were unable to break through.
After some fine back play, it was ironic then that the breakthrough came from a spilt ball, Millard picking up a loose pass and finishing in clinical style.
Evans' conversion struck the upright though the try looked to have been enough to have secure the win.
But Sir Gar refused to lie down and when Morgannwg conceded a needless penalty, replacement Jones stroked over what looked to be the winning kick.
It proved to be just that though in a thrilling finish, Evans could have won the game with the last kick of the match.
Happily for Morgannwg though, his touchline effort was wide and Sir Gar were crowned champions.
The Colleges final was the culmination of a successful day of schools and colleges rugby at the Millennium Stadium. Castell Nedd School were crowned Schools Under 11 champions while Rhondda Schools got the better of Pontypridd Schools in the Dewar Shield final.
Ysgol Glantaf were 20-12 winners in the Under 18 final before Sir Gar became Colleges League champions.
You can watch highlights of today's feast of rugby on S4C tonight at 11.10pm.