Despite leaving influential midfielder John Heslin, who was eligible again after suspension, on the bench, Westmeath were in control for much of the first half and with Conor Lynam converting three frees, they sailed into an advantage of 0-5 to 0-1 after 21 minutes.
Longford, who were missing centre-forward Padraig Gill, who was sidelined with an ankle injury, took a while to get going on the Navan pitch, where they shocked Meath two weeks earlier.
However, they finished the half strongly with points from John Keegan, James McGivney (free) and promising minor Robbie Smith, leaving the Lake County holding a 0-5 to 0-4 interval lead.
Longford kept up the good work on the restart with a levelling point from James McGivney before substitute Peter Reynolds gave them the lead for the first time on 37 minutes.
Westmeath brought Heslin into the action early in the second period but he was shown a straight red card on 49 minutes following a clash with Kevin Diffley.
The Lake County side made light of the loss to lead by 0-9 to 0-7 with eight minutes left. However, Longford, seeking a first title in the grade, made the extra man tell from then to the finish.
After Padraig McCormack had the sides level for the fifth time, McGivney secured the win from a free.
P Columb; J Carroll, F Battrim, A Quinn; S Mahon, C Smith, D Cooney; K Diffley, J Keegan (0-1); P McCormack (0-1), J McGivney (0-4, three frees), M Mulvey; R Smith (0-2, one free), D Gallagher, JJ Matthews (0-1).
Subs: P Devine for Carroll (13 mins), P Reynolds (0-1) for Gallagher (21), C Keegan for Mulvey (47), J Noonan for Smith (56), D Gallagher for Mahon (60).
E Carberry; B Moran, K Sheridan, K Maguire; M McCallon, K Martin, J Dolan; D Keenan, D Daly; A Corroon (0-1), C Lynam (0-6, six frees), G Egan; I Coffey (0-1, free), C McCormack, T McDaniel (0-1).
Subs: J Heslin for Keenan (33), J Gilligan for Corroon (52).
Referee: B O’Shea (Dublin).