Making 23 substitutions throughout an entertaining and at times tense O'Byrne Cup semi-final encounter, the Dessie Farrell era ultimately got off to a losing start for Dublin.
The new Sky Blues boss, with old ally Mick Galvin notably by his side, presided over his first competitive game which concluded in drama at Glennon Brothers Pearse Park.
Dublin had led by two points with three minutes of normal time remaining but conceded a three-in-a-row of Longford points, two of which were scored by Darren Gallagher with his second the match winner in the 74th minute.
That came from a free for a foul on Gallagher himself and referee Alan Coyne was booed off the field at the end with two Gardai intervening to keep control of the situation.
It was unexpected drama and intensity at the end of an early January game that Longford just about edged to keep their hopes alive of a rare piece of silverware.
Farrell was without all of his 2019 All-Ireland winning squad though Stephen Cluxton did exit the dressing-room afterwards having apparently travelled with the group.
The players involved were mainly rookies and former underage talents, many of whom played under Farrell when he was Dublin minor and U-21 manager with some of them winning All-Irelands in those grades.
Niall O'Callaghan scored two goals for Dublin and the younger brother of senior star Con hinted that he may be a chip off the old block. Dublin's other goal was scored by Killian O'Gara, younger brother of former senior forward Eoghan who retired after last September's five-in-a-row success.
Dublin's main squad won't resume training until Tuesday so this was an opportunity for new players to impress and CJ Smith got up from corner-back to kick the first point of the Farrell era.
O'Gara netted in the 19th minute to level it at 1-1 to 0-4 but moments later was substituted as Farrell operated a curious strategy of rolling players on and off. Promising defender Cian Murphy was substituted twice in the game.
Longford, meanwhile, made just three changes in the tie as they tried to get to grips with an ever changing Dublin challenge.
The sides remained level at half-time, 1-3 to 0-6, and O'Callaghan struck his first goal seconds after restart, a left-footed finish following good work by Dermot McLoughlin.
His second goal arrived in the 50th minute and left Dublin with a two-point advantage that they retained until that late trio of Longford scores arrived which settled it in favour of the hosts.
Longford will play Offaly - who beat Westmeath by 0-16 to 0-11 - in the final next weekend in what will be a midlands derby with Padraic Davis' side hoping to claim the title for the first time since 2000.
Longford scorers: R Brady (1 free) and D Gallagher (4 frees) (0-5 each); K Diffley (1-0); O Kenny (0-2); D Doherty (0-1).
Dublin scorers: N O'Callaghan (2-0); K O'Gara (1-0); G Seaver (0-2); CJ Smith, D McLoughlin, G O'Reilly and N Doran (0-1 each).
Longford: P Collum; P Fox, A Farrell, B O'Farrell; I O'Sullivan, M Quinn, CP Smyth; D Gallagher, K Diffley; G Rogers, D Mimnagh, D Reynolds; R Brady, J Hagan, O Kenny.
Subs: D Doherty (0-1) for Rogers 57, P Lynn for Kenny 60, L Connerton for Reynolds 68.
Dublin: M Shiel; CJ Smith, E O'Brien, D Byrne; N McGovern, G Hannigan, C Murphy; E O Conghaile, T Lahiff; N Walsh, D O'Brien, G O'Reilly; K O'Gara, N O'Callaghan, C Sallier.
Subs: R McGowan for Murphy 18, S Fulham for O'Gara 19, R Deegan for O'Reilly 21, S Cleary for Smith 25, G Seaver (0-2) for Walsh 25, D Quinn for Sallier 26, D McLoughlin (0-1) for O'Brien 27, N Doran (0-1) for Lahiff 29, E O'Dea for McGovern 31, C Howley for O Conghaile 34, O Conghaile for Lahiff 41, O'Reilly for Deegan 45, Walsh for Seaver 45, Murphy for Cleary 49, Sallier for O'Callaghan 50, O'Brien for Quinn 55, O'Gara for Lahiff 55, McGovern for McLoughlin 55, Lahiff for Doran 60, Seaver for Fulham 60, Smith for Hannigan 63, O'Callaghan for Sallier 68, O'Dea for Murphy 73.
Ref: A Coyne (Westmeath).