Johnson shot 7 of 8 from the floor and hit five 3-pointers. Tevin Prescott added 15 points, Andre Smith chipped in 14 with six assists, and Keon Moore finished with 13 points for Winthrop (17-12, 12-6), which will play its first game of the tournament on Friday.
Presbyterian (10-21, 6-12) went up 5-2 after Markus Terry hit a 3 and Jordan Downing made two free throws, but Winthrop took over from there. The Eagles led by 14 points at the half and held a 67-43 advantage with 6:18 remaining.
Downing led the Blue Hose with 15 points, scoring 12 in the first half. Presbyterian, which holds the No. 8 seed in the Big South Tournament and plays on Wednesday, shot just under 30 percent overall.