Elijah Pughsley hit three 3s and finished with 13 points for Western Carolina (5-8). Torrion Brummitt had 12 points and nine rebounds.
Neither team scored until Justin Browning hit a pair of free throws more than four minutes in, sparking a 12-3 run by the Catamounts.
The Lions scored 13 in a row, including a 3 by Patrick Monroe to open the second half, to trim their deficit to 35-31 but Harrelson answered with a 3-pointer that ignited a 10-2 spurt and Western Carolina led by at least nine the rest of the way.
Michael Carson led Mars Hill with 17 points and Monroe added 15.
Western Carolina outrebounded the Lions 54-35, including 21-11 on the offensive glass.