The 38-year-old, who held an overnight four-shot lead, fired a closing three-under-par 69 to defeat South Africa's Charl Schwartzel, who signed off with a 72, at the Amata Spring Country Club.
Westwood's 37th career victory, thanks to a 22-under-par 266 aggregate, will see him move up one rung to second place on the Official World Golf Ranking when it is released on Monday.
Masters champion Schwartzel mounted a brief challenge, narrowing the lead down to three on the back nine with an eagle on the 11th hole but a bogey on 14 snuffed out his challenge as Westwood cruised home in the Asian Tour's final tournament of the 2011 season.
While Westwood's triumph was expected, it was a day of drama for the Tour hopefuls with South African Keith Horne securing 61st place on the Order of Merit to claim the final Tour card for 2012 ahead of Filipino teenager Miguel Tabuena by a mere US$250.
Westwood did not produce the fireworks that helped him to a career low 60 in the opening round and a 64 on the second day which matched the Asian Tour's 36-hole record of 20-under-par but he played solidly enough to complete a commanding wire-to-wire victory and end his year on a winning note.
"It means a lot. Any win does. It's special this week because of the difficulty of the golf course, and whom I was up against in the weekend. Charl is the Masters champion and he's had a great year. 20 under after two rounds, it's yours to lose really with an 11 shot lead. There was pressure involved. I was pleased with the way I played today," said Westwood, who earned US$158,500.
Schwartzel got to within three of Westwood after a superb eagle on 11 which the Englishman three putted for par. The challenger rolled in another long birdie putt at the next hole to put the pressure on but Westwood responded superbly with a birdie of his own and restored his four-shot lead with his fourth birdie of the day on 13, which gave him great satisfaction.
The contest ended when Schwartzel, playing his ninth straight event, dropped a bogey on 14 after missing the green and Westwood stamped his class with another birdie on 15 in front of massive galleries at Amata Spring.
"The way I putted on the back nine was great. It means a lot, gives me a lot of confidence. I've been working on these for the past four or five weeks and it was time to hole some of these under pressure," he said.
Schwartzel lamented his bogey on 14. "I pulled him back to within three shots after 11 holes. It felt pretty good especially against a player like Lee. All credit to Lee. He made two great birdies on 12 and 13. I had a very bad lie on 14 and made bogey. That was a big swing there. It was getting close but I gave it a go. I'm happy with my performance," said the 27-year-old.
"If it wasn't for Lee, I would have probably won the tournament."
Michael Thompson of the United States claimed third place while local star Chawalit Plaphol finished tied fourth to emerge as the top Asian finisher alongside England's Simon Dyson on 276.
Rookie Tabuena shot a closing 76 for tied 39th position to miss out on a full card for next season by a hair's breadth as Horne, who finished in 61st place, earned the last card. Dutchman Guido Van Der Valk jumped from 83rd to 59th after finishing a creditable sixth with a 73.
The leading Australasian in the tournament was Scott Barr who finished in 18th position while the next best were Mark Brown and Kunal Bhasin in 27th.
David Gleeson led the Australasians on the finalised Asian Tour money list finishing in 6th position with US$292,000.
Scott Barr was 26th, Darren Beck 27th and Scott Hend 28th, Marcus Both 42nd, Rick Kulacz 46th and Tim Stewart 56th rounding out those who finished inside the all important top 60.
Scores after round 4 of the Thailand Golf Championship being played at the par 72, 7,453 Yards Amata Spring CC course (a- denotes amateur):
266 - Lee WESTWOOD (ENG) 60-64-73-69.
273 - Charl SCHWARTZEL (RSA) 69-66-66-72.
274 - Michael THOMPSON (USA) 69-66-69-70.
276 - Chawalit PLAPHOL (THA) 73-71-65-67, Simon DYSON (ENG) 69-70-68-69.
279 - Daisuke MARUYAMA (JPN) 73-68-68-70, Guido VAN DER VALK (NED) 71-69-66-73.
280 - Alex CEJKA (GER) 71-71-71-67.
281 - Jeev Milkha SINGH (IND) 71-68-72-70, Tetsuji HIRATSUKA (JPN) 71-72-68-70.
282 - Gregory BOURDY (FRA) 68-71-72-71, Prom MEESAWAT (THA) 72-68-69-73.