Wilmington (4-8), conversely, shot 39.3 percent (22 of 56) from the field, including five straight misses during a 15-0 Miami run over a 2 minute, 23 second span in the second half.
Harris capped the run with a 3-pointer and Johnson added two free throws to give Miami a 69-55 lead with 9:53 left to play.
Wilmington, which led five times, never got any closer as the Quakers missed 14 of their last 18 shots.
Brandon Candella led Wilmington with 16 points.
Maryland point guard Melo Trimble, considered a lock to leave the program before struggling in the second half last season, has withdrawn from the NBA draft and will return for his junior season.
Louisville's Chinanu Onuaku, who recently underwent a medical procedure to correct a heart rhythm issue detected during the combine, will keep his name in the NBA draft pool.
Andrew White III will return to the Nebraska men's basketball team next season after removing his name from the NBA draft.