It's Mason County fifth straight loss, dropping the Royals' record to 2-7.
Nate Kaiser rushed for 252 yards and a touchdown and Austin Knipp passed for 170 yards and two touchdowns as Grant County snapped a seven-game losing streak.
Grant County scored on its first three possessions and led 21-7 at the end of the first quarter and 35-13 at halftime.
Carson Brammer scored a touchdown on a quarterback sneak in the third quarter and Seth Chambers capped off a six-play, 60-yard drive with a three-yard touchdown run with 5:27 left in the fourth quarter, but Grant County was able to run out the clock to seal the victory.
Chambers rushed for 181 yards and three touchdowns on 29 carries.
Brammer completed 8-of-22 passes for 109 yards.
Grant County outgained Mason County 489-304 in yards of total offense.
Mason County closes out the season next Friday night at home against the Lewis County Lions.
GC 1st Q 10:30 Nate Kaiser 36 yard run (Cody Trenkamp kick) GC 7, MC 0
GC 1st Q 8:05 Damian Ingguls 40 yard pass from Austin Knipp (kick failed) GC 13, MC 0
MC 1st Q 5:52 Seth Chambers 3 yard run (Brock Gilligan kick) GC 13, MC 7
GC 1st Q 4:54 Caelin Nix 4 yard run (Austin Knipp run) GC 21, MC 7
GC 2nd Q 9:06 Austin Knipp 6 yard run (pass failed) GC 27, MC 7
MC 2nd Q 3:41 Seth Chambers 5 yard run (kick failed) GC 27, MC 13
GC 2nd Q 00:43 Damian Ingguls 15 yard pass from Austin Knipp (Austin Knipp pass from Damian Ingguls) GC 35, MC 13
MC 3rd Q 3:09 Carson Brammer 1 yard run (Brock Gilligan kick) GC 35, MC 20
MC 4th Q 5:27 Seth Chambers 3 yard run (Brock Gilligan kick) GC 35, MC 27
