Senior guard Pete Webb also had 22 points for the Saints (9-16), and added five rebounds and four assists. Fellow senior Juwuane Sandridge had eight rebounds and three steals.
Martinsburg held a 14-11 lead after the first quarter, but the Saints outscored the Bulldogs 26-19 in the second period. Brewington scored 11 points and Webb added seven. Freshman Michael Wright had seven points for Martinsburg (16-5).
The Saints scored the first 14 points of the second half - nine on a trio of Brewington 3-pointers - to build an 18-point lead.
Martinsburg managed to get within striking distance in the fourth quarter after a pair of Jason Dirting baskets reduced the deficit to 57-51 with 4:23 remaining, but the Saints outscored Martinsburg 16-10 the rest of the way.
"It was a nice victory," St. James coach Wayne Ridenour said. "I thought we did a real good job of executing as far as getting the kind of shots that we wanted. We only had one bad stretch towards the end of the third quarter. Other than that, I thought that we played well."
Sophomore forward Larry Scott added 17 points, nine rebounds and four assists for the Saints. Brewington had six assists and five steals.
Dirting had 19 points, seven rebounds and six assists, while Wright added 16 points and 15 rebounds.