The former world number one, leading by a stroke ahead of compatriot Andy Sullivan, carded a bogey-free final round of four-under-par 68 for a four-day total of 18-under.
Austrian Bernd Wiesberger (67), South African Louis Oosthuizen (68) and Belgian Nicolas Colsaerts (70) finished tied for the second spot at 11-under.
Once the 27-year old Sullivan found water at the second hole for a triple bogey seven, there was no catching Westwood, whose last triumph came at the 2012 Nordea Masters in Sweden.
Sullivan fought back with a hat-trick of birdies from the fourth but suffered an astonishing meltdown after that as he went on to drop six more shots to finish tied 13th with a final-round 78.
Play was suspended for over four hours at the Kuala Lumpur Golf and Country Club due to lightning threat but it provided no respite for the chasing pack as Westwood returned to sink two more birdies on the 13th and the final hole.
The 40-year-old, who finished seventh at the Masters last week, credited the recent changes he had made to his game for the turnaround.
"I started working with a new coach a few weeks ago, Mike Walker, and Billy Foster came back on the bag at the end of last year, so I was going back to what I had done before because it had worked," Westwood said after winning the $2.75 million Asian Tour co-sanctioned event.
"It's started to work already - the last couple of weeks I've played well in Houston and at the Masters last week and this week I've obviously played very well.
"It's a golf course that suits my game; it's very tight in certain areas. I played well, I putted well, and the short game is good."
- Sports & Recreation