Taking a three-shot advantage over fellow Briton and Ryder Cup team mate Ross Fisher into the final round, Westwood maintained plenty of distance between himself and the chasing pack all day.
He made sure of the victory with an eagle three at the par five 12th before closing with a three-under 69 and four-round tally of 269, 19-under.
It was his 22nd European Tour success and his third at the tournament after lifting the trophy in 1996 and 2001.
"I played really well this week, you don't win tournaments by five shots without playing well," Westwood said.
Fisher carded a one-under par 71 on the final day to finish second at 14-under with Spain's Sergio Garcia producing a five-under par 67 to rocket up the leaderboard to end at 13-under in a share of third alongside Finn Mikko Ilonen and Swede Peter Hanson.
The eyes of the golf world now shift to San Francisco with the year's second major beginning on Thursday.
Westwood is paired with defending champion Rory McIlroy and world number one Luke Donald for the first two rounds.