Marist qualified for a semi-final showdown against All-Ireland champions St Mary's Edenderry with a one-point win in a bad-tempered game in Castledaly.
Knockbeg were leading by 0-4 to 0-2 when their centre-forward Aaron Forbes was shown a straight red card following an off-the-ball incident in the 17th minute.
Despite this, Knockbeg still held the lead at the break (0-5 to 0-4).
The winners' profligacy continued on the change of ends and, despite limited possession, two frees from Cormac Murphy had Knockbeg ahead by two points (0-7 to 0-5) by the end of the third quarter.
However, the Athlone lads struck for three fine points via Robbie Ashe, Niall McInerney and Peter Ryan to take the lead.
Knockbeg were then reduced to 13 men when their goalkeeper Danny Bolger was sent off.
Despite missing a brace of glorious goal opportunities, Marist retained their narrow lead.
In the last minute of normal time, Athlone substitute Andy O'Sullivan became the third player to pick up a straight red card.
There was still time for some more drama as, with the last kick of the contest, Knockbeg's Michael Keogh tried to blast home a goal from a 25-metre free only for his effort to fly over the crossbar.
Scorers – Marist Athlone: N Neary 0-3 (2f), A Byrne 0-2 (1f), N McInerney, N Kelly, R Ashe, P Ryan 0-1 each. Knockbeg College: C Murphy 0-4 (4f), JJ Mahon, C Hogan, M Keogh (f), D Tompkins 0-1 each.
Marist College Athlone – T Finnerty; B O'Donoghue, E Quigley, M McArdle; J Marshment, C Marlowe, R Ashe; A Curley, S Omokuro; M Monaghan, N Neary, TJ Fox; N Kelly, N McInerney, A Byrne. Subs: D King for Marshment (10), P Ryan for Monaghan (48), A O'Sullivan for Fox (57).
Knockbeg College – D Bolger; L O'Sullivan, D Brophy, K Doyle; A Campion, D Ramsbottom, A Brennan; JJ Mahon, C Hogan; M Keogh, A Forbes, D O'Reilly; E O'Neill, D Tompkins, C Murphy. Subs: D McDermott for Doyle (25), D Lalor for O'Neill (49), D Halpin for Murphy (55), A McGrath for McDermott (57).
Ref – P Neilis (Meath)