KAVARNA, Bulgaria, May 19, 2013 (AFP) - McDowell had been 2 down after just four holes to Thongchai but sensed near the turn his 43-year- old rival was starting to tire in the hot and cloudless conditions on the Thracian Cliffs course.
The Northern Irishman, who had beaten South African Branden Grace earlier in the day to reach the final, fought back to draw level with Thongchai at the 12th before going 2up in winning the 14th and 15th holes before sealing victory with a par at the 17th.
"It's just unbelievable to win as it's been a long week, and it could be said these events are tougher to win than stroke-play events," said McDowell.
"Somehow beating six guys head-to-head with the standard of golf required there is certainly an amount of feel good factor.
"However I thought when Thongchai went 2up after four holes that this wasn't the script I had read.
"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 on that."
McDowell also becomes the first Irish-born winner of the tournament and 12 years after Padraig Harrington lost the 2001 final to Ian Woosnam.
"To be the first Irish-born winner of the event is special as it to join some of the greats of the game who have won this trophy," said McDowell.
"But I'm looking at the trophy and seeing Montgomerie, Els, Westwood, Palmer, Player, Norman, Els. The history of this event is just fantastic.
"We talked a lot this week what Volvo have done with this event, what they have done with golf around the world and coming here to Bulgaria this week was a brave move.
"Anyone looking at TV pictures from this week and seeing just how spectacular Thracian Cliffs then I think it's paid off."
Victory has moved McDowell up one place to number seven in the world and also eight places to the head of the Race to Dubai money list.
Thongchai secured the biggest prize cheque of his European Tour career of 400,000 euros.
The Thai golfer, who had defeated South African Thomas Aiken in the semi-final, has jumped 24 places to seventh on the Race to Dubai.
"I am very happy and had a very good week," said Thongchai.
"I made only one mistake in the final on the 14th hole after the 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, and I enjoyed coming here for a first time in my career to Bulgaria."