Matthew Dellavedova added 11 points and six assists for the Gaels (15-4, 4-1 West Coast), who never trailed.
St. Mary's held Portland to 43 points in the schools' previous meeting, on Feb. 23, 2012.
Paul McCoy made his long-delayed debut with Saint Mary's, with one rebound in three minutes.
The senior guard and Portland native transferred from SMU after the 2009-10 season, sat out the following year per NCAA rules, then missed all of last season after suffering a second knee ACL tear. He averaged 13.4 points as an SMU freshman in 2009-09.
Mike Krzyzewski says he has good reason to be excited for next season thanks to a combination of Duke's incoming freshmen and a core of experienced upperclassmen.
Georgia guard Juwan Parker has been awarded a medical redshirt, leaving him with two years of eligibility, after missing the 2015-16 season with a partially torn Achilles tendon.
Forward Trevon Bluiett has withdrawn from consideration from the NBA draft and will return to Xavier for his junior season.