Lauren Nicholson added 12 points for the Gaels (9-1), who forced 24 turnovers that led to 27 points. They shot 47 percent in the second half.
Cassie Harberts had 17 points and 12 rebounds for the Trojans (5-5), who shot 52 percent in the first half, but they had 15 turnovers and trailed by two at the break.
St. Mary's used an 8-0 run to push its lead to 11 points with 5:55 left. Harberts scored four straight points to get USC within seven. But the Trojans were forced to foul after again falling behind by double digits.