Thomas sealed his fifth PGA title in 2017 -- and his first of the new 2017-18 PGA season -- when he birdied the second extra hole and Leishman made bogey after finding the water.
The pair went to a sudden-death playoff after completing the first PGA Tour regular-season event in South Korea tied at 9-under 279.
Thomas, who shared the overnight lead with Scott Brown, fell behind when he had a double bogey on the 550-yard par-5 No. 3 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."
Thomas regained the lead and looked set for the win in regulation until he bogeyed No. 17, a 195-yard par 3 hole. But he made amends with a birdie at the last to join Leishman in the playoff.
"I really played some great golf after the third hole but it's just such a long day with these conditions," Thomas said.
Thomas said he is looking forward to some time off after his breakout season.
"I'm so excited to not do anything," Thomas said. "I officially have nothing left in the tank at this moment."
Leishman, who ended a 5-year drought on the PGA Tour when he won the Arnold Palmer Invitational in March, then added the BMW Championship in September, looked to have slipped out of contention when he made bogeyed the 5th, 7th and 8th holes to tumble down the leaderboard. But he birdied No. 9, No. 12 and No. 14 to join Thomas at 9-under then finished bogey-birdie to get in the playoff.
Replaying the par-5 18th, the pair both made par on the first extra hole before Leishman gambled and lost on his next attempt at the 568-yard final hole, finding the water after trying to reach the green in two.
"Obviously, disappointing to finish off that way. I probably got a little bit of a lifeline on the first play-off hole and I didn't take advantage of it. You give one of the best players in the world a chance like that, he's probably going to take it. Disappointed but second is not all bad," Leishman said.
"It 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."
Cameron Smith of Australia finished one stroke back in third place at 8-under with Whee Kim, the highest placed finisher of the 16 South Korean players in the field, finishing fourth at 7-under after closing with a 72.
Thomas' five tournament wins this calendar year included his first major, the 99th PGA Championship, which he won in August.
"I'm so pumped," Thomas added. "I probably wasn't very fun to be around those first two days. I was just glad I finally got back to myself being patient these last two days."