Grantham hit 9 of 16 shots, including a pair of breakaway slam dunks, as he played all 32 minutes. He was the only Bulldog to score more than seven points.
It marked the fourth victory of the season for Martinsburg over its backyard rival.
The Patriots suffered eight of their 13 turnovers in the first quarter and shot 26 percent in the first half in falling behind by double digits. But guard Dominique Newman scored nine of his 15 points in the second half to get things rolling.
Washington actually had two chances to tie in the final minute.
After Bell's 3 from the corner to make it 44-42, Martinsburg spread the floor to run clock, but Jordan Robinson thought he had an open path to the basket and darted in for a layup. He was whistled for charging into Maleke Jones, however, and Washington got the ball with 18 seconds remaining.
Newman's driving attempt missed, Jalen Lewis rebounded for Martinsburg and was fouled with 7.7 seconds to go. He made the first but missed the second, leading to Washington's last-ditch try from Bell.
"I said before the game if we kept it in the 40s and 50s, we had a chance to win,'' said Patriots coach Don Bullett, "and we had a chance to win in the fourth quarter. Like [Thursday], we got off to a slow start and didn't do a very good job against their pressure . . . but for the most part we kept battling, and that's why we were still in the game.
"If someone beats you four times, they're better than you, but I still think on a given night we can beat them. I'm happy with our effort down here. We played two games and competed in two games. We've got everybody back next year - we played two juniors and four sophomores - so next year, you'll see an even better team.''
Reach Rick Ryan at 304-348-5175 or rickr...@wvgazette.com.