Thomas and Leishman finished regulation play at the CJ Cup locked at nine-under after both players birdied the 18th on Sunday.
But after squaring the first playoff hole at Nine Bridges, Thomas claimed the winner's prize of $1.66 million when Leishman bogeyed the second.
It was a see-sawing finish with Leishman and Thomas both having eagle opportunities at the last to win outright.
They'd get to know the 18th particularly well as it would feature as the two playoff holes.
Leishman recovered remarkably from a wayward tee shot on the first playoff hole, taking a drop and threading his second between trees to stay in contention with a par.
But it wasn't to be when they re-visited the par-five, Leishman dropping a shot after finding the water.
Thomas, who last month claimed a $10 million payday at the Fed Ex Cup, refused to layup and finished just short of the green before claiming a birdie and the title.
The world No.4 had shared the overnight lead with Scott Brown but fell behind when he had a double bogey on the 550-yard par-five third hole.
"I just kept telling myself it was a bad golf swing," Thomas said. "It wasn't a mental error or wasn't a wrong judgment that we made but just a bad golf swing at a pretty bad time but I knew you are going make bogeys out here today."
The silver lining for Leishman was a cheque for $999,000 and a projected career-high ranking of world No.12.
Leishman said he was disappointed with the conclusion but wanted to go down swinging. "I probably got a little bit of a lifeline on the first play-off hole and I didn't take advantage of it," he said.
"It (the second playoff hole) was a bit of a dodgy lie. I was certainly not going to lay up. I just chunked it a little bit and goes in the water. If you go down, you want to go down like that. You don't want to go down laying up and making par. I rather attack and try to take it. I would certainly sleep better at night just doing that."