Bel Air coach Valerie Cooper said her goal was to hold C.M. Wright's Erin Updegrove to under 20 points.
Visiting Bel Air held Updegrove, the area's leading scorer with a 23.2 average, to 14 points, but fourth seed C.M. Wright held the Bobcats to under 10 points in each quarter to win the Class 3A, North Region championship, 45-29.
Updegrove led the Mustangs (17-8) in scoring, and Kellie Flynn led the sixth seed Bobcats (13-12) with nine.
The win gave the Mustangs their first regional title, and advanced them to Thursday's Class 3A state semifinals at UMBC.
"This was our goal at the beginning of the season, and I think our focus really showed today," said C.M. Wright coach Gloria Leidlich.
"I think this was a harder game for us than Randallstown [which the Mustangs beat in the semifinals] because the kids know each other so well."
The game was tied at nine early in the second quarter, but Wright scored 11 of the next 17 points to take a 20-15 lead at the half.
The Mustangs converted 10 of their first 11 free throws to help widen the margin.
The Mustangs went on a 10-5 run in the third quarter to put the game away.
"Our defense and foul shooting was the difference," said Leidlich.
"We tried several different defenses, and we switched and helped each other out well."
C.M. Wright forward Annie Sappington said she thinks the Mustangs might be peaking at just the right time.
"We've starting working better as a team lately," said Sappington.
"No one thought we could beat Randallstown, and we believed in ourselves and now we're champions."
Cooper, whose team had won 12 of its last 14 after a 1-9 start, said defense cost her team the game.
"They did a good job taking us out of our game," said Cooper.
"They slowed us down, and we flourish when we can run."
The Bobcats were limited without center Jackie Hinson, who injured her ankle in Thursday's win over Franklin.
She played little and scored only two points.
The Mustangs will enter the state tournament as long shots, but that doesn't faze Leidlich.
"I've been going to [the tournament] for 20 years as a spectator, and I know that what happens there really doesn't matter," she said.
"We'll play as hard as we can, but just getting there is an accomplishment. This is the first [girls basketball] team here to win a regional title, and I'm proud of them."