Thongchai won the American Family Insurance Championship in Wisconsin by one shot for his 22nd career title, after eight victories in European and 13 on the Asian Tour.
Tied for the lead overnight, the 52-year-old Thai legend signed off on 4-under 68 for a winning total of 14-under 202 at University Ridge Golf Club. His maiden victory on US soil came on his 19th career start in the 50-and-over PGA Tour Champions.
Tom Pernice Jr finished second after a final round of 66. Miguel Angel Jimenez, who was tied overnight with Thongchai, shot 70 to share third at 12-under with Jerry Kelly (67), Marco Dawson (67) and Kirk Triplett (68).
At the trophy presentation, Thongchai revealed he was able to deal with the pressure effectively thanks to his training in the Thai military, which he joined at 16 before becoming a paratrooper at 20 years old.
Royal Thai Army service had also allowed him to hone his golf skills at the military club, until he turned professional at the age of 30.
"I think life in the military helped me a lot as my mental state was built up during military exercises," he said.
Thongchai rose to world No 27 at the height of his career but is now ranked No 1,181 after not playing a major tournament for several years.