Longford missed a number of chances before David O’Sullivan rose highest form a free-kick to make it 0-1 on the half hour.
Despite being outplayed for spells, Harps were given the perfect opportunity to draw level in the 68th minute when they were awarded a penalty when Pat McCann as impeded inside the area.
However Town goalkeeper Bennion did enough to save from Sean McCarron.
Long made just of the points ten minutes later when Mark Salmon scored from inside the area.
Longford go up and harps will be left to ponder how a season, which started quite brightly, fell away from them.