The Gamecocks (14-4, 3-2 Southeastern Conference), who defeated No. 24 Florida on a shot at the buzzer Wednesday night, went on a 26-5 run against the Rebels that started with the last basket of the first half and carried over to the first eight minutes of the second.
Downey hit a 3-pointer with four seconds left in the first half to give the Gamecocks the lead for good at 37-35.
Mike Holmes and former Josey standout Dominique Archie then led a charge in which South Carolina scored the first 11 points of the second half. Archie finished with 12 points.
The Rebels (10-9, 1-4) missed their five field goal attempts in second half, before David Huertas hit two free throws. Mississippi didn't make a field goal until the 11:23 mark when Huertas hit a 3.
Huertas scored 25 points for Mississippi.