No. 5 seed San Francisco (20-12), which beat Portland on Wednesday, next gets WCC No. 1 seed Saint Mary's (25-5) in Saturday's opening semifinal.
The Dons took the lead for good at 47-46 on a layup by Cole Dickerson with 11:26 left.
Two free throws by Anthony Ireland and a 3-pointer by Jarrod DuBois brought the fourth-seeded Lions within 61-58 with 1:39 left, but they came no closer.
Ashley Hamilton had 14 points and eight rebounds, and Ireland finished with 13 points for Loyola Marymount. The Lions posted their highest victory total since the 1989-90 team won 26 and reached the NCAA tournament regional finals.