More than 100 minutes of football were played as both sides fought with tigerish intensity.
Three players Rory Kavanagh and Graham Norton of Maynooth and Sligo's Eamonn McGee were red-carded and a total of eight others were given yellow cards by Michael Collins.
No prisoners were taken in an energy-sapping contest and, remarkably, we could have been treated to an unprecedented third period of added time had Sligo not scorned a gilt-edged opportunity to even matters again at the death.
The defending champions, with centre-forward Paul Finlay virtually unerring from placed balls, opened up a two-point lead at the end of the first quarter.
But Maynooth, who also had a gem of a free-taker in full-forward Tadgh Brosnan from Meath, came back to level matters at four points apiece at half-time.
With the wind to their backs Maynooth opened up after the resumption and scorched into a three points lead. However, sparked by the scoring exploits of Finlay, who kicked six points, Sligo showed champions do indeed die hard.
Facing defeat they stormed back from the brink, and when full-forward Nicky Joyce nudged them in front by a point with normal time almost up, Maynooth looked doomed. But Brosnan came to their rescue with a great equalising point from a free under intense pressure in stoppage time.
It was 0-8 to 0-8 but within 20 second of the start of the first period of extra time Sligo looked to have struck a crucial blow when Christy Toye set up Joyce for a fisted goal. Maynooth faced a searching examination. But they came through with flying colours. With Brosnan scoring three points on-the-trot, they were level again at 1-8 to 0-11 at the break.
Brosnan, continuing on his admirable scoring streak, edged Maynooth in front again with a point. But Finlay countered to leave it level again at 1-9 to 0-12 at the end of the first period of extra time.
Maynooth drew first blood in the additional 10-minutes with a point from Declan Lally in the 93rd minute. His golden strike proved to be the matchwinner at the end of this extraordinary marathon.
As the clock ticked over the 100-minute mark, Sligo's Christy Toye soloed through but the Donegal star opted to go for goal and when his shot was brilliantly held by outstanding goalkeeper Conor Breman, Maynooth were through and the champions were out.
Scorers for NUI Maynooth: T. Brosnan 0-7 (0-5 from frees); D. Lally 0-3; R. Murrelly, P. Hurley and A. Dillon 0-1 each.
Sligo IT: N. Joyce 1-1; P. Finlay 0-6 (0-5 from frees); B. Moloney and B. Boyle 0-1 each.
NUI MAYNOOTH: C. Breman; D. Geraghty, M. O'Se, M. Foley; G. Norton, J. Keane, C. Moran; P. Hurley, R. Kavanagh; A. Dillon, K. Ennis, D. Lally; R. Munnelly, T. Brosnan, B. Moran.
Sub: E. Walsh for Munnelly.
SLIGO IT: P. Durcan; A. Higgins, P. Kelly, J. Jordan; B. Dunnian, E. McGee, K. Cassidy; B. Boyle, P. Brady; M. Moyles, P. Finlay, C. Toye; M. Brehony, N. Joyce, B. Moloney.
Subs: G. Pearson for Brehony; D. Barden for Moloney; M. McKeever for Dunnion; R. Haran for Brady.
Referee: M. Collins (Cork).