Both players finished on nine-under-par 279 for the four rounds of the Tour's first regular season event in South Korea, a shot better than Cameron Smith in third, with home hope Kim Whee in fourth spot, two strokes further back on six-under.
Thomas closed with an even par 72 while Leishman picked up two shots with a 70 in the final round.
After they carded matching pars at the first extra hole, the 18th, Australian Leishman leaked his second shot into the water right of the green as the pair came down the par five closing hole for a third time on Sunday.
Despite seeing his opponent's ball splash down in the lake, Thomas opted against laying-up and kept faith with a wood, battering his second from 243 yards just short of the green.
The FedExCup champion's eagle putt would come up short but his birdie was more than enough to beat Leishman, who finished with a bogey six.
"It was tough out there today, for these last three days it was extremely difficult condition with the wind but, I’ve stayed really patient this weekend and patient today," the 24-year-old said.
"It felt really good to hit two really good shots there on 18 in regulation and I had to putt with a chance to win it.
"I hit two great 3-woods there at that second play-off. It's a great way to cap off the year. I’m really excited to do nothing for a while," he added.
Scott Brown (76), the overnight co-leader with Thomas, endured a final round to forget with back-to-back double bogeys at the fourth and fifth derailing his title hopes, while Anirban Lahiri (74) was leading after going two-under through 13 holes, but dropped four shots over the closing five holes.
Both players finished on five-under 283, tied for fifth place with Jamie Lovemark (71), Brian Harman (71), Luke List (72) and last week's CIMB Classic champion Pat Perez (68).
(Reporting by Peter Rutherford; Editing by Amlan Chakraborty)
Share
