Dwayne Brown Jr. and Quinn Taylor scored 15 points apiece and the Aggies (13-11, 6-5 Mountain West Conference) shot 56.4 percent from the floor. McEwen had a career-high six of Utah State's 13 3-pointers.
The Lobos closed to 70-64 after scoring 10 straight, then to 83-80 with 1:03 left after Troy Simons' dunk, but got no closer. The Aggies sealed it with four free throws in the final 27 seconds.
Taylor's layup put Utah State up 28-19, but the Lobos led 32-30 after Anthony Mathis' jumper capped a 13-2 run. Brown hit a 3 and Utah State led 39-35 at halftime.
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.