Brown's layup and short jumper gave the Catamounts (8-12, 3-4 Southern) an 80-75 lead. Harrelson's jumper and three Brown free throws made it 84-78 before Harrelson added two more free throws with 17 seconds left for a seven-point edge.
Harrelson's right-hander off the glass with 1:19 left was the final basket in the first overtime.
Western Carolina led by 10 with 1:16 remaining in regulation but Jestin Lewis, with 27 points, scored nine points for Mercer (14-6, 4-3) including a four-point play to tie with 35 seconds left.
Mercer led by eight at halftime but the Catamounts shot 57 percent in second half.