Harps created the better chances in an open first-half, yet Longford looked the more composed outfit and led at the break. They went close early on when Gary Merritt headed wide from a good position before Longford took the lead on 13 minutes when Karl Bermingham slotted the ball home.
And although Harps created chances through Matthew Harkin and Paul McVeigh, Longford doubled their advantage on 32 minutes when Gary Shaw headed home.
But the Donegal side were given a lifeline two minutes before the break when Harkin rose to head home a Thomas McBride cross.
McVeigh shot narrowly over early in the second-half and Aaron O’Hagan then headed just wide as Harps enjoyed their best spell.
However, luckless Harps then conceded a penalty as O’Hagan fouled Jamie Duffy. Colm James scored at the second attempt, after the spot-kick was initially saved by Ciaran Gallagher.
FINN HARPS: Gallagher; Doherty, O’Hagan, McMonagle, Coll; Harkin, McBride, McGowan, Merritt; Bonnar; McVeigh. Subs: McGinty for McVeigh (77 mins), Cowan for Harkin (80 mins), Duffy for McBride (88 mins).
LONGFORD TOWN: Hyland; Haverty; Hope, Deans, Lee; Walsh, Gillespie, James; Shaw, Bermingham, Shaw. Subs: Duffy for Walsh (63 mins), Purdy for Shaw (85 mins).
Referee: Mr A. Reale (Dublin).