UNC Greensboro's RJ White made a layup with 3:56 left in the extra period to give the Spartans a 75-74 lead.
But Torrion Brummitt's 3-point play on the ensuing possession regained the lead for Western Carolina and sparked a 9-2 run for the Catamounts to close out the win. All six of the remaining points came at the foul line, with Justin Browning hitting 3 of 4.
Harrelson sent the game into OT with a 3 that tied the score at 72 with 34 seconds left. The Spartans had a chance to win on the final possession, but Kayel Locke turned it over.
Brummitt finished with 20 points for Western Carolina (7-9, 2-1 Southern Conference).
Marvin Smith led UNC Greensboro (5-12, 1-3) with 21 points.