The American held a comfortable four-shot lead heading into the final round and showed tremendous composure at TPC Summerlin to card a five-under-par 66 and seal his first success on the PGA Tour since triumphing at the US Open last year. "It means the world," said Simpson. "This year, I felt like I've gotten better, just hadn't got the win. We've been working hard."
Simpson, who finished ahead of Jason Bohn and Ryo Ishikawa, made an excellent start to his round, pitching to 15 feet for birdies at the second and third holes. He made another birdie to enter the turn on three under for his round, but then three-putted at the 12th for his only bogey of the day, which left him just three strokes clear of compatriot Bohn. Simpson bounced back at the next hole, hitting his second shot to within a couple of feet before further birdies at the 15th and 16th provided a safeguard.
"I was playing good, solid golf, I didn't look at the leaderboard all day but the course was drying out and I knew it was playing tough," he added.
Simpson's overall 24-under-par score ensured he becomes the fourth player in a row to triumph at this tournament with a score of at least 20 under par. The 28-year-old has often flattered to deceive following his maiden major win at Olympic Club in 2012 and he admitted to initially struggling to cope with expectations placed on him.
"I did a little bit," he said. "I knew there was a lot of pressure on me when I moved up the world rankings and I knew a lot of people start looking at you and watching you every week. I felt like I learnt to deal with it towards the end of the year and I felt a lot better mentally this year."
Scotland's Russell Knox slipped back to a tie for 30th place, finishing disappointingly with a 73 following rounds of 67, 65 and 69.
Royal Porthcawl will stage the Amateur Championship in 2016, the seventh time the Welsh venue has hosted the championship, and the first time since 2002. The news is timely for Royal Porthcawl, which is already hosting next year's Senior Open and which will be visited by R&A officials next month to determine its suitability to host an Open.