Monaghan had a 10-0 lead at half time and looked almost certain to win the game before Letterkenny stole it at the death through Peter Scott's drop goal.
It was a dramatic conclusion to a tense final that could have gone either way between two extremely evenly-matched sides.
Letterkenny had started the day well, with a difficult wind swirling around Kingspan Stadium forcing them to keep the ball in hand early on.
Several phases of strong carries worked them into the Monaghan 22 where they won an early penalty on the 5-metre line and looked poised to strike. However, a handling error allowed Monaghan to turn it over and clear the danger.
Some smart kicking from hand by Monaghan out-half Gareth Graham kept the Letterkenny defence guessing, but nothing came of their early forays into the 22.
It wasn’t until the 26th minute that the sustained pressure finally told, and Monaghan number 8 Christopher Brady drove over for the opening try of the final. Gareth Graham added the conversion to make it 7-0.
5 minutes later, Graham nudged over a penalty from 35 metres to keep the scoreboard ticking over in favour of Monaghan as the first half drew to a close.
After the break, Letterkenny got their first points on board when Ben Galbraith hit back with a penalty with 50 minutes on the clock.
It sparked a bit of life back into the side, and they became more open and expansive with possession as they tried to get back into the final.
They then spent long periods camped in the Monaghan 22, where a resolute defence absorbed phase after phase of relentless Letterkenny attack from the forwards. This intense pressure only eased off when a costly knock-on gave Monaghan the chance to clear the danger.
It wasn’t long until Letterkenny were back on the attack though, winning a late penalty and kicking for the corner. The resulting lineout saw the forwards retain the ball and maul over from close range with Keenan Barrett touching down to make it 10-8.
It gave Galbraith a chance to draw level with a tough conversion, but his effort cannoned back off the posts with just a few minutes left to play and it looked like Letterkenny had missed their chance.
However there was still time for one last roll of the dice, and after a final few desperate phases inside the Monaghan 22, the ball found its way into the hands of out-half Peter Scott, who calmly slotted the winning drop goal with the final act of the game.
### Monaghan RFC
(15-9) Lorcan McGillan, Carl Foentjis, Andrew McPhillips, Evan Rutledge (c), Simon O'Connell, Gareth Graham, James Allen
(1-8) Jamie Sherlock, Bernard O'Reilly, Richard Carleton, Rory McKenna, David O'Callaghan, Daniel McGregor, Jason McGuirk, Cristopher Brady
Replacements: Ciaran McKenna, Marc McArdle, Clayton Wilson, Darren Berry, Dermot McGlone
### Letterkenny RFC
(15-9) Daniel Faulkner, Ben Galbraith, Jack Speer, Andrew Gibson (co-c), John Anthony McFadden, Peter Scott, Ben Hopkins
(1-8) Barry Laverty, Shaun McGill, Cristopher Digimas, Jack Kennedy, Ciaran Clarke (co-c), Ben Scott, Keenan Barrett, David Brown (co-c)
Replacements: Calum Moore, Cormac McMenamin, Martin Molloy, Kaymin Shortt, Eoghan O'Riain