Briana McNutt's potential game-winning layup just before the fourth quarter buzzer was waved off by by an official who called McNutt for traveling
Clark County led by 10 points at halftime and took its biggest lead at 39-26 with 6:16 left in the third quarter.
Mason County closed the gap to five points at 43-38 at the end of the third quarter.
A.J. Reed hit two free throws to tie the score at 48-48 with 3:31 left.
Jordan Frodge made a reverse a layup to give the Lady Royals their first and only lead at 50-48 with 2:19 to play.
Clark County guard Kennedy Igo hit two free throws to tie the game with 1:33 left.
After the traveling call, one second was put back on the clock. Clark County, inbounded the ball, but did not attempt a shot.
Igo opened the overtime period with a three-pointer.
Whitney O'Mara made one of two free throws with 2:42 to play.
A basket in the low post from Malena Bell gave the Lady Cardinals a 55-51 lead two minutes left.
O'Mara hit a jumper off the dribble with 1:35 to go.
Shemaya Behanan pushed the lead back out to three by making one of two free throws with 18 seconds left.
Mason County missed three shots in the lane in the closing seconds.
Clark County will play 13th Region champion Harlan in the first round of the St. Elizabeth Healthcare/KHSAA Girls Sweet Sixteen Basketball Tournament Wednesday at 8 p.m. at BB&T Arena at Northern Kentucky University.
Igo scored a game-high 22 points.
Behanan added 13 points.
Frodge led Mason County in scoring with 15 points.
O'Mara followed with 13 points and Reed tossed in 11 points.
CC (29-4) J. Flowers 8, Igo 22, Bell 8, Behanan 13, T. Flowers 3, Hardiman 5
MC (23-9) O'Mara 13, Reed 11, McNutt 8, Frodge 15, Boone 6
CC 13 22 08 07 06- 56
MC 09 16 13 12 03- 53