The Gaels clinched at least a share of the West Coast Conference regular-season title and will capture the undisputed crown with a win over San Francisco Saturday.
St. Mary's hit 50 percent from the field and made 20-of-25 from the foul line while improving to 24-5. The Gaels are 13-2 in conference play.
Portland was held to two points over the final 8:51 of the first half while the Gaels were building a 35-18 advantage at the break. Both Rob Jones and Matthew Dellavedova had 12 for the Gaels.
Portland (6-22, 3-12) was led by Nemanja Mitrovic with 14 points.