The Northern Irishman advanced to the final by defeating Branden Grace of South Africa 3 and 2 in the semifinals. He needed the the qualities that allowed him to play on two Ryder Cup-winning teams.
Having fought back to win No. 7, McDowell appeared to feel Thongchai was tiring and made his move to win Nos. 12, 14 and 15th.
"I thought when Thongchai went 2 up after four holes that this wasn't the script I had read," said McDowell. "But then after winning the seventh and managing to halve nine and 10, I felt good. I felt like I was starting to get in control and I sensed Thongchai was weakening, and I just sensed an opening and I just seized that opening."
Thongchai's check for 400,000 euros was the largest of his European Tour career, 28,000 euros more than he earned for winning last year's Wales Open.
Thongchai, who defeated Thomas Aiken of South Africa 3 and 2 to reach the final, has jumped 24 places to seventh on the Race to Dubai.
"I am very happy and had a very good week," Thongchai said. "I made only one mistake in the final on the 14th hole after a poor drive. But it was a good tournament for me. My game is very solid. "I came here of course to win, and to come second I am very proud and satisfied as it is a good result for me.
"I did get very tired this afternoon, but more importantly I enjoyed the tournament."
- AP