The host Tar Heels improved to 3-1 and Maryand fell to 2-1.
McCool assisted on the game-tying goal with 22 seconds left in regulation.
The loss halted the Terps' 56-game regular-season winning streak.
For Maryland, Sophomore Karli Hartshorn posted a career-best 11 draw controls while adding two goals to pace Maryland, while juniors Jen Giles (Mount Hebron, three goals, one assist) and Caroline Steele (Severn, two goals, two assists) added four points apiece.
"You're obviously never happy with a loss, but I thought this was an outstanding early season game with a great deal of intensity," said Terps coach Cathy Reese. "Credit UNC, they really put a lot of pressure on our defense," said Terps coach Cathy Reese. "We had far too many turnovers and need to value possessions more. We have things to work on, but have faced back-to-back quality teams on the road and we're only three games into the season. We'll watch the film, make adjustments and are excited to get back home and in front of our fans this week."
After going down 3-1 early, Maryland rattled off six straight goals over a 14-minute streak, five on free position, to take a 7-3 lead. Senior Taylor Hensh (Marriotts Ridge) led with two goals over that span.
North Carolina then scored three straight before Maryland responded with goals by junior Meghan Siverson and senior Megan Whittle (McDonogh) to make it 9-6. At halftime, the Terrapins led by one goal, 9-8.
The Terps used two more free position shots early in the second by Giles and Siverson to take an 11-9 lead with 25:19 to go.
Both defenses tightened up after that, creating turnovers and missed shots. With 8:42 left, Giles completed her hat-trick to put Maryland up 15-13. The Tar Heels would get one back with 3:51 left and then tie it up with just 22 seconds left to force overtime. About a minute into the overtime period, North Carolina clinched the home victory.
Mount St. Mary's 14, Army West Point 10: Taylor Carhart (Manchester Valley) had five goals to lead the host Mount (2-1) past the Black Knights (2-2). Mount St. Mary's scored seven goals per half.
No. 14 Penn 12, Johns Hopkinns 11: Freshman Zoe Belodeau had four goals, including the game-winner, to lead the host Quakers (2-0) past the Blue Jays (3-1).
Belodeau scored the game-winner with 52 seconds left to play. Nicole DeMase had a game-high five goals for Hopkins.
UMBC 21, Virginia Commonwealth 9: Sara Moeller (St. Paul's) had a game-high six goals and six assists to power the host Retrievers (3-0) past the Rams (0-2).
Caitlin Miller scored the first two goals of a 4-0 run for UMBC. Miller, Lauren McDonald (Catonsville) and Taylor Oliver (St. Mary's) all finished with three goals.
No. 3 Boston College 22, Brown 12: The host Eagles (5-0) went on a 15-6 run in the second half to beat the Bears (2-1). Kaileen Hart (5 goals, 5 assists) has 100 career points and Sam Apuzzo broke the school's single-game draw control record with 11.
Virginia Tech 17, No. 10 Duke 14:Tristan McGinley had a game-high six goals as the host Hokies (3-2, 1-0 ACC) upset the Blue Devils (3-1, 0-1).
No. 16 Virginia 14, No. 12 Princeton 10: Avery Shoemaker had four goals to lead the visiting Cavaliers (3-0) over the Tigers (1-1). Virginia went on a 7-4 run in the second half.
Yale 14, No. 19 Massachusetts 9: The host Bulldogs (3-0) jumped out to an 11-5 lead and beat the Minutewomen (1-2). Kayla Duperrouzel scored a game-high five goals for Yale.