An electrifying run of five birdies in six holes around the turn put Clark a stroke in front of the chasing pack on a treacherous day for scoring.
He coolly parred the last six holes for a flawless five-under-par 67 and a 16-under total of 272 to triumph by one shot in the tournament widely considered the "fifth major" by the players.
Australian Robert Allenby birdied two of the last five holes for a 70 to finish second while world number four Westwood had to settle for a tie for fourth at 12-under after dumping his tee shot into water at the 17th en route to a 74. US Open champion Lucas Glover covered the back nine in a scintillating five-under for a closing 70 and third place at 14-under.
Clark, a perennial bridesmaid on the US circuit who had finished runner-up eight times in 205 career starts, fired the day's lowest round for his long-awaited breakthrough.
"I came out today and showed what I can do on the golf course,” said Clark.
