Harrison put the clamps on Hillgrove, holding the Hawks to just four points in the first quarter, and earned a spot in tonight's 8 p.m. title game against third-seeded McEachern, a winner over Campbell in Friday's other semifinal.
It will be the first region championship game for Harrison since 1996, when it beat South Cobb in the finals of the Region 5AAAA tournament - which was also played at McEachern.
"It's very exciting for us," Harrison coach Robert Churchwell said. "We're going to try to stay at this level (of play), but it's going to be a tough game."
Marshall Guilmette scored 13 points to lead Harrison, while Jamal Watson added 10.
The Hoyas also had five blocked shots, three from Elijah Staley and two for Guilmette.
Marcus Dortch scored 17 points for Hillgrove (16-10), which will play Campbell in the 4 p.m. consolation game.
Defense was the deciding factor for Harrison in the first quarter when the Hoyas jumped out to an 18-4 lead. For Churchwell, coming out strong on defense was the main focus for his team coming into the game.
"For the last day-and-a-half, we have been emphasizing defense, defense, defense," he said. "We played a good team defense, and our defensive stops gave up some opportunities offensively."
Harrison continued to build its lead throughout the second quarter, and the Hoyas were up 38-19 at halftime.
Guilmette and Ryan Blumenthal combined for 13 of Harrison's 15 points in the third quarter - with Blumenthal hitting two 3-pointers - and the Hoyas increased their lead by as much as 25 points before ending the period with a 49-28 advantage.
The fourth quarter was more of the same for Harrison, which led 58-35 with 1:54 left in the game before Dortch hit a 3-pointer and Rhodis Eason made one of two free throw attempts to help Hillgrove narrow the margin at the end.