Mayhew had half of Portland State's eight 3s in the first half and the Vikings were 9 of 14 from close range, and opened a 44-32 lead. Montana State never got closer in the second half.
Tyler Hall had 16 points with four treys for the Bobcats (11-7, 4-1), with Konner Frey and Keljin Blevins adding 12 each.
Portland State finished with 14 3-pointers, its second best effort from distance this season.
Montana State had 20 assists on 24 baskets but also had 21 turnovers.