In Friday's MHSAA Class 3A state semifinal against Bailey, Leach hit two free throws to give Booneville a 58-55 lead with 39.5 seconds to play.
Iury Dixon then missed a 3-pointer and Leach finished the job with a miss and a make to the relief of Booneville's rowdy student section in a 59-55 win.
The reigning state champs improved to 32-3.
"Everybody has moments where you don't have to play great, you've just got to win," Booneville coach Michael Smith said. "They did just enough to survive and got a stop or two when they needed it and took years off our life, but..."
Booneville led 49-42 in the final quarter before giving in to some late pressure. With 1:43 left, the Blue Devils led 56-52 after a pair of Kenny Paul Geno free throws.
But after one free throw make in three tries for Bailey, a steal and bucket from Dixon cut that lead to 56-55 with 60 seconds to play.
"We knew we were going to be in a dog fight," said Bailey coach Timothy Speech. "It was five men on each side playing as hard as they could to get to a state championship."
Geno led the Blue Devils with 22 points and 10 boards. He was 6-for-6 in the opening half. Leach finished with 14 and Keldrick Lesley had 13.
Leach saved his best for the fourth quarter, including a 3-pointer for a 47-42 lead. It was then Geno to his right for a 49-42 advantage, tying Booneville's largest lead. Leach then answered a Bailey three-point play with a bucket for a 51-45 lead.
Bailey (22-11) got 18 points from Dixon and 15 from Jau-Quaylon Wilson but only 11 from leading scorer Lee Montgomery.
"I've never coached a kid that I'd rather have at the free throw line than Darius Leach," Smith said. "If he misses those shots, I've got peace because he's got ice water when it comes to that. I just trust him that much."