Oregon State (5-1, 3-0 Pac-12) won its fifth game in a row since a shocking opening loss to FCS Eastern Washington.
The home loss dealt a blow to Washington State's (4-3, 2-2) hopes of reaching a bowl for the first time since 2003.
Washington State led 24-17 late in the third quarter when the Beavers erupted for three quick touchdowns to take control of the game.
Mannion completed 34 of 51 passes and was intercepted once. Brandin Cooks caught 11 passes for 137 yards and two touchdowns, and ran for another. Richard Mullaney caught five passes for 122 yards.
Washington State quarterback Connor Halliday completed 26 of 49 passes for 248 yards as the Beavers were able to shut down WSU's potent pass attack. He was intercepted three times.