Winthrop led 34-33 at the half. The Eagles went on an 11-0 run to build a 50-39 lead. It was 58-47 after Winthrop made 10 of its first 17 field goals of the second half and the Eagles led by 20 with 6:49 to go following a 12-0 run.
Ethan Kay scored 15 points on five 3-pointers and Davon Bell added 15 points, on 11-of-12 shooting at the foul line, with eight assists for Presbyterian (8-13, 1-7). Reggie Dillard had 13 points, Romeo Crouch 11.
Mannion, who recently reclassified and is ESPN's No.11 overall player in the class of 2019, chose Arizona over Marquette.
He'll fit in with Sean Miller's plans for a faster tempo on offense.
The cream of the crop of American high school basketball will congregate in Colorado Springs from Oct. 5-7, as USA Basketball unveils its newly expanded Junior National Team minicamp in collaboration with the NBA, NCAA and NBPA.