Sub Tiernan Flanagan bagged two goals as Derry ended Monaghan's reign as Ulster U-21 champions at Inniskeen.
Monaghan surged into a three point lead, before a Jamie Donaghy goal brought Derry level. Three unanswered points from Hagan helped the visitors to a 1-5 to 0-5 interval lead.
Although David Garland had a penalty saved by Ben McKinless,Monaghan soon drew level.
Derry took charge when a point attempt from sub Tiernan Flanagan dropped into the net. Flanagan got his second goal four minutes later, and Conor Feeney pushed the lead out to seven.
Although David McAlister pulled back a goal from a penalty, Derry secured victory with points from Jordan and Dean Curran as they advanced to a semi-final clash with Armagh.
Scorers - Derry: T Flanagan 2-0, P Hagan 0-5 (2fs), J Donaghy 1-0, C Feeny 0-3, J Curran, D Curran 0-1 each. Monaghan: D McAlister 1-2 (1-0 pen), M Bannigan (1f), F Maguire, F Hanratty 0-2 each, P Finnan, D Gleeson, D Garland 0-1 each
Derry - B McKinless; N Keenan, O Duffin, C McGrogan; P Kearney, M McEvoy, C Mulholland; J Doherty, J Curran; J Donaghy, P Hagan, C Doherty; B Grant, C Feeney, D Tallon. Subs: P Coney for Donaghy (23), T Flanagan for Grant (H-T), R McElwee for Tallon (46), D Curran (0-1) for Kearney (BC, 57).
Monaghan - S Garland; C Lennon, M Murnaghan, J Daly; B Kerr, J Mealiff, S Finnegan; S Woods, P Finnan; N Kearns, M Bannigan, D Gleeson; F Maguire, D McAlister, D Garland. Subs: F Hanratty for Woods (H-T), S McCabe for Gleeson (41), R McDonald for D Garland (45), J Walshe for Finnan (50), D Ward for Maguire (52).
Ref - R Barry (Down).