But instead it was they who came with a late surge at Markievicz Park to snatch victory from a Roscommon side bidding for their third Connacht U21 title in a row.
Roscommon led by six points after eight minutes, were five up at the break and having never trailed were still two ahead with just over three minutes remaining.
And then Mayo struck.
Substitute James Carr sent a delivery in from the left wing and Liam Irwin, who battled back from injury to play, got a powerful fist to sent the ball into the bottom right corner.
Roscommon were stunned but while man-of-the-match Diarmuid Murtagh got them back on level terms, Mayo scented victory and duly delivered, working the ball upfield for Michael Plunkett to shoot the winner two minutes into stoppage time and set up an All-Ireland semi-final against Dublin.
“Better late than never I suppose,” said Mayo manager Mike Solan.
“We were under pressure for most of the game, but thankfully we were able to work things around in the first 15, 20 minutes of the second-half, claw Roscommon back a little bit, push ourselves back into a position where we could compete.”
He paid tribute to Irwin for delivering the key score. “Liam is just a born winner. He has worked himself to get himself into brilliant shape. He has had a bit of an injury setback a few weeks ago, he worked incredibly hard to get back.”
Roscommon, bidding for their 10th crown and a first ever underage three in a row, were left to rue a series of missed opportunities.
Murtagh gave them a perfect start with a goal after three minutes, Henry Walsh should have added another, but they were well worth their 1-6 to 0-4 interval lead.
Mayo came back after the restart in search of a 24th title and first since last appearing in the final in 2009, but while Roscommon remained in front they did not kill the game and left themselves vulnerable, not helped by Cathal Compton’s injury and a black card to Sean Mullolly.
“There was probably a lot of things in the game that probably went against us at different stages,” said Roscommon manager Mark Dowd.
Obviously, losing a couple of the lads, Sean Mullolly, Cathal Compton, was a big blow. They would have been the engine room of our team. We dug deep right to the very end, but that’s the way these games go,”
L Irwin 1-3 (0-2f, 0-1 ‘45), C Loftus 0-3 (0-3f), F Duffy 0-2, M Plunkett 0-1, F Boland 0-1, B Reape 0-1.
D Murtagh 1-4 (0-2f), R Daly 0-3, U Harney 0-1, B Stack 0-1, H Walsh 0-1.
M Flanagan; D Kenny, S Cunniffe, E Doran; M Hall, S Coen, S Akram; D O’Connor, M Ruane; M Plunkett, C Loftus, F Boland; L Irwin, B Reape, F Duffy.
E O’Donoghue for Doran (18), J Carr for Loftus (45).
R Fahy; J Connaughton, S Mullolly, K Beirne; F Lennon, U Harney, B Stack; T O’Rourke, C Compton; K Farrell, H Walsh, B Kelly; D Murtagh, H Darcy, R Daly.
M Daly for Mullolly (30), R Raftery for Compton (36), C Callinan for Kelly (46), K Smith for Walsh (58).
J Molloy (Galway).