With the teams deadlocked following an explosive final quarter, substitute Mangan popped up to swing over the match-winning point from distance.
It capped off an enthralling finale that saw Mercy Mounthawk’s Ciarán O’Connell goal with just 10 minutes left to drag the Tralee school level (1-4 to 0-7).
Less than a minute later Mercy Mounthawk crept ahead when Jack Barry pointed from close-range but St Colman’s responded quickly after Martin Brennan was fouled and awarded a penalty, following a superb Philip Blackburn run.
Dramatically, midfielder Anthony Spillane — who kicked five points and performed well in midfield alongside his brother Colm — watched his low effort well saved by Tralee goalkeeper Ciarán O’Connor.
But after an inspirational effort from Gary O’Callaghan drew St Colman’s level, the Fermoy school cranked up the pressure, pinning Mercy Mounthawk back before working the space for Mangan’s late winner.
The Kerry school had one last chance to force extra-time but an effort dropped short. The losers will be haunted by such wastefulness — in the first half they had six wides and dropped four efforts short. They trailed 0-3 to 0-2 at half-time despite having the better chance.
Mercy Mounthawk restarted with a quick Darragh Carmody brace to edge ahead 0-4 to 0-3, before scores from Colm Spillane and three Anthony Spillane frees put St Colman’s 0-7 to 0-4 in front before the dramatic late conclusion.
Scorers for St Colman’s: A Spillane 0-5 (4f), G O’Callaghan 0-2, C Spillane, D Mangan 0-1 each.
Scorers for Mercy Mounthawk: C O’Connell 1-0, D Carmody 0-3 (2f), C Fitzgerald, J Barry 0-1 each.
ST COLMAN’S: F Hogan; M Kennedy, C Barry, T Carroll; P Murphy, T Condon, P Blackburn; C Spillane, A Spillane; R Bransfield, P De Roiste, A Fenton; M Brennan, G O’Callaghan, N McNamara.
Sub: D Mangan for A Fenton (53).
MERCY MOUNTHAWK: C O’Connor; F Horgan, P McMahon, J O’Kelly; D McGarry, J Cahalane, A Byrne; M O’Sullivan, J Barry; JJ Prior, C O’Connell; D Griffin; D Carmody, C Fitzgerald, D O’Brien.
Subs: E Sheehy for JJ Prior (42); C Sayers for D Griffin (42).
Referee: Michael Meade (Limerick).