Western Carolina trailed by as many as eight points in the first half, but Sinclair scored seven of Mercer's 10 points to open the second half, during a 10-4 run, to take a 33-31 lead.
Mike Brown had 12 points and Rhett Harrelson added 11 for Western Carolina (9-11, 4-4 Southern). The Catamounts were 8 of 23 (34.8 percent) from the field in the first half and finished the game at 44.4 percent.
Darious Moten hit nine free throws and finished with 18 points for Mercer (11-10, 6-2). The Bears were held to 16 field goals on 54 attempts and saw their three-game winning streak snapped.
Lawrence Brown cut Mercer's deficit to 55-50 with 36 seconds left, but Sinclair answered with a dunk.
Mercer led 27-23 at the half behind Moten's 15 points.