Poulter, who becomes the first Englishman to have won any of the four individual World Golf Championship titles since their inception in 1999, scooped a cheque for $1.4m (£906,000) and a career-high world ranking of No5.
Casey, who was also beaten in last year’s final, earned a second prize of $850,000 (£550,000) and takes sixth place in the rankings – the third Englishman in the top six.
“It feels really, really nice,” said Poulter of his first win on American soil. “It’s been a long time coming and I am very happy. It was a good day’s golf.
“I knew I was in great form and I felt calm all day – nerves didn’t play any factor at all. I know Paul’s disappointed being runner-up two years in a row.”
Before he could even contemplate teeing off against Poulter, Casey had to see off Camilo Villegas in an epic semi-final that had run into a second day. Having played five extra holes on Saturday without a result, Casey needed just one yesterday morning to finally defeat the Colombian and reach the final.
Once there he encountered the flamboyantly-attired Poulter clad from top to toe in pink – and in superb form. Casey led briefly when he was conceded an eagle at the second, but from then on Poulter took charge. The Hertfordshire player birded the next, went ahead at the seventh and by the 14th was four clear. Casey stayed in the hunt by reducing the gap to two before the end of the first round, but never got closer.
Poulter sealed a landmark victory at the 34th when he holed from 10 feet after Casey – runner-up last year to Geoff Ogilvy – failed to get up and down from a bunker.