Mickelson, a sophomore center, played only five minutes in the first half after picking up a pair of early personal fouls. He made up for his absence after the break, scoring 10 points in the period's first 6:33 to give the Razorbacks the lead for good.
Paced by Mickelson's perfect 6-of-6 second half shooting, Arkansas made 67 percent of its field goal attempts after halftime and led by as many as nine points.
Robert Morris (8-5) stayed close with its free-throw shooting, connecting on 15 of 18 attempts after halftime.
The Colonials pulled within three points on a pair of Lucky Jones free throws with 2:50 left, but could get no closer.