Mark Quigley converted a controversial 88th minute penalty as Sligo Rovers won their first title in 35 years at the Showgrounds.
Two Raffaele Cretaro goals on 21 and 24 minutes had Sligo in a commanding position, but there was real drama as the afternoon evolved.
Christy Fagan and Chris Forrester responded with goals for the visitors, but Quigley earned the victory Sligo needed at the death.
In a furious finale Sligo's Danny Ventre and Saints' Seanie O'Connor were red carded at the end of an absorbing encounter.
Eager to make a positive start Sligo were quickly on the front foot as they seized the initiative in front of a passionate home crowd.
In the centre of the park Danny Ventre provided the bite with Lee Lynch and Joseph Ndo allowed to influence the play further up the park.
Sligo crafted a string of chances in the first quarter with a Mark Quigley snap shot that drifted wide in the sixth minute signalling their intentions.
Ross Gaynor and Lynch also went close for the Bit O'Red, while Conor Kenna had to be alert when making a last ditch challenge to thwart Quigley.
Despite the misses Sligo ploughed on defiantly eventually registering a deserved 21st-minute goal following a splendid move.
Quigley's glorious touch set it in motion with Gaynor and Pascal Millien involved with Raf Cretaro steering the ball under the advancing Barry Murphy.
Three minutes later Sligo's afternoon got even better when Gaynor's corner was flicked on by Gavin Peers with Cretaro steering home a header at the back stick.
Sligo were two up, the Showgrounds was rocking and a 35-year-old search was on the verge of being terminated.
The Saints improved slightly as the interval approached with Chris Forrester and Christy Fagan going close, but Sligo still departed armed with a two-goal advantage.
Eight minutes after the restart Pat's trimmed the deficit when Seanie O'Connor's delivery caused panic in the Sligo defence and Christy Fagan rifled the visitors back into the reckoning.
The woodwork came to Sligo's rescue in the 58th minute when O'Connor's free kick rebounded off the crossbar as Pat's pressed for an unlikely leveller.
That is precisely what happened on the hour mark when Forrester, now operating effectively in the middle, unleashed a powerful effort, which skimmed off Sligo left full Gaynor before crossing the line.
It was tight and tense and Millien dragged an attempt narrowly wide with 15 minutes left on the clock as Sligo sought to pinch a third.
Saints substitute Anto Flood dashed away from Jason McGuinness, but his sweet right-footed strike hit the bar as the visitors came within a whisker of adding a third.
But it was Sligo who achieved that in controversial fashion when referee Damien Hancock awarded a penalty deeming that Conor Kenna had handled a Peers shot.
The title was on the line, but Quigley stepped up to convert the spot kick with ice cool composure and Sligo sealed the deal.
Late on there was time for even more talking points with Sligo's Ventre and Saints' O'Connor dismissed following an injury-time altercation.
Hancock's last whistle was greeted with a pitch invasion as Ian Baraclough's stylish Bit O'Red outfit gleefully celebrated a stunning success.
Sligo Rovers: Rogers; Henderson (Cawley, 54), Peers, McGuinness, Gaynor; Lynch, Ventre, Ndo (Buchanan, 84); Millien (Dillon, 76), Quigley, Cretaro.
St Patrick's Athletic: Murphy; O'Brien, Kenna, Browne, Bermingham; Bolger (Faherty, 46), Chambers (Russell, 70), Carroll; O'Connor, Fagan, Forrester (Flood, 80).
Referee: D Hancock (Dublin).