During that time Clark suffered no fewer than eight runners-up finishes, but his patience was finally rewarded with a brilliant closing 67 as the Sawgrass course really bared its teeth.
Westwood, second at The Masters Tournament last month to a last-day 67 from Phil Mickelson and bogeyed the last to finish one behind at The Open Championship last July, was trying to become Europe's third successive winner of this title.
He led by one both at halfway and after three rounds and shared top spot with nine to go, but could not respond to Clark's mid-round charge - five birdies in six holes.
The Worksop golfer came to the last two holes knowing he needed to birdie both just to force a play-off on 16 under par.
Instead, however, the World Number Four went into the water on the famous near island green 17th, double-bogeyed it and a par on the last for a 74 left him in a tie for fourth, while Clark was one of only two players to break 70.
After making an eight foot par putt on the last he had to wait to see if anybody could catch him.
With Westwood falling out of the hunt Australian Robert Allenby was the last man in with a chance.
He had a chance to draw level with an eagle on the long 16th, but left that on the lip and did the same with an 11 foot birdie attempt at the 17th.
Another birdie was therefore needed to take it to sudden death, but the 462 yard 18th was a beast all day into the wind and Allenby missed the green and parred.
It left him in second place on his own, with US Open Champion Lucas Glover third and Westwood in a tie with American quartet Davis Love, Heath Slocum, Bo Van Pelt and Ben Crane.
Clark said: "I did all I could - that's as good as I could have played.
"I figured I was right there once I got to 15 under (on the 11th) and I felt like I hit every shot I wanted to."
Westwood twice lost the lead to Allenby in the early stages, the first of them when the Melbourne golfer chipped in for eagle from deep rough on the second.
But Europe's leading player birdied there and thanks to an 18 foot birdie putt on the fifth and a 22 footer for par at the long ninth - he had chipped over the green into a bunker - he turned one in front.
By then, though, Clark was on a real purple patch. He had holed from ten feet at the seventh and when four more came in a row from the ninth he was one in front.
That became two when Westwood bogeyed the 14th after an errant drive, but although he came back from that with a 48 foot putt on the next it was only for par and he lipped out for birdie at the 16th.
Luke Donald and Graeme McDowell, who had been in the hunt at halfway, finished with rounds of 75 and 74 respectively to be tied for 26th on five under.
Oliver Wilson had seven bogeys in his last 12 holes for a 79 and one over, while Swede Robert Karlsson put two balls in the water for a quadruple bogey nine at the 11th as he also returned a 79.