Washington finished with 10 points and 10 rebounds, Jordan Downing added 15 points and both recorded six boards to help Presbyterian (5-16, 2-5) outrebound Longwood 33-26.
Down 61-51 at the 10:58 mark, Longwood went on a 9-0 run capped by two free throws from David Robinson to make it 61-60 with 7:05 remaining.
The Blue Hose answered with a 15-4 run, taking a commanding 76-65 lead on Mutakabbir's layup with just 1:32 left to play.
Presbyterian made four free throws and Longwood (3-18, 0-7) converted just one field goal in the final minute.
The Blue Hose completed all 16 of their free throws for the game.
Robinson and Michael Kessens scored 16 apiece to lead the Lancers, who have lost 11 straight games.