In a game that saw three ties and two lead changes, Clay Chastain's 79-yard touchdown pass to Kyle Holmes with 5:29 left was the difference.
"I thought Kennesaw Mountain came out ready to play," Harrison head coach David Hines said. "They came out and got after us but what I was really impressed with was that our kids finished it. We've had a hard time doing that this year and that's why I'm proud of the kids."
At the 6:36 mark of the fourth quarter, Kennesaw Mountain tied the contest at 21-21 with a five-play, 63-yard scoring drive when Nick Bell pulled in a 6-yard touchdown pass from Dallin Finley.
Following a scoreless third period, the Hoyas concluded a 33-yard drive that took eight plays with a 4-yard touchdown run by Zach Moore, putting Harrison on top, 21-14, with 7:47 remaining in the fourth quarter.
Down late in the first half, the Hoyas scored a touchdown with just 8 seconds left before halftime to tie the score at 14-14. A 4-yard pass from Chastain to Justin Myricks capped off the four-play, 44-yard scoring drive.
Harrison started quickly, putting together a 51-yard scoring drive on the first possession of the night. On the fourth play of the drive, Moore found the end zone on a 6-yard run, putting the Hoyas ahead, 7-0, with 10:27 to go in the opening quarter.
"The kids fought hard, as they have most weeks, but we just can't seem to get over the hump," Kennesaw Mountain head coach Chad McGehee said.
The Hoyas' second lost fumble of the opening period, recovered by the Mustangs' Ryan McGrail, led to Kennesaw Mountain's first score. It took the hosts four plays to cover 44 yards with Finley hooking up with Xavier Ealy on a 13-yard touchdown pass, making it a 7-7 contest with 5:26 remaining in the first period.
After driving from its own 35-yard line to Kennesaw Mountain's 30, Harrison came up short on a 47-yard field goal attempt, keeping the score tied at 7-7 early in the second quarter.
Kennesaw Mountain took its first lead of the game after moving 60 yards in 10 plays. Finley's 3-yard touchdown pass to Devin Putmon gave the Mustangs a 14-7 advantage with 1:11 to go in the second period.
For the Hoyas, Chastain finished 14-of-23 passing for 250 yards while Moore rushed 10 times for 74 yards. Holmes had five receptions for 138 yards.
Finley completed 14 of 24 passes for 233 yards for the Mustangs. Christian Fleming was his favorite target, catching five balls for 109 yards.