MVSU (4-26, 4-13 Southwestern Athletic Conference) has won three of its last seven after starting the season with 20 straight losses. Grambling (15-14, 11-5) has lost two straight after an 11-game winning streak.
The Tigers led by as many as 10 points in the first half, but MVSU used a 26-8 run for a 45-37 lead going into the break.
MVSU led 56-40 early in the second half, but Grambling State cut it to 73-71 with 2:15 left after a bucket by Ivy Smith Jr. However, that would be as close as the Tigers would get down the stretch.
The teams combined for 101 rebounds after missing 92 of 147 field-goal attempts.
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.