After watching major wins at the US Open and PGA Championship slip through his fingers on the final day this year, Johnson did not falter a third time.
The 26-year-old carded a rock-solid final round two-under-par 69 on a brilliant late summer afternoon at Cog Hill Golf and Country Club for a winning total of nine-under 275.
“It was a great day, to finally get it done, after all the things I’ve been through this summer,” a beaming Johnson told reporters. “To get it done on Sunday, I can’t feel any better.
“I’ve had a few mishaps but today, to come back and get it done, I’m really proud of myself.”
Casey had led by three strokes but watched in horror as his advantage evaporated with three straight bogeys from the 13th and he finished with a 69 and second place.
Americans Ryan Moore (73), who held a one shot lead going into the final round, Matt Kuchar (72), the FedExCup leader heading into the season-ending Tour Championship, Kevin Na (70) and South Korean K.J. Choi (69) all finished two shots further back in a tie for third on 278.
Despite winning his second title, it has been a season a agonizing near misses for Johnson.
He came frustratingly close to notching his first major victory at the US Open at Pebble Beach, where he held a three-stroke lead going into the final round before shooting 82.
Last month at the PGA Championship, Johnson was left crushed when he was handed a two-stroke penalty for grounding his club in a sand hazard on the 72nd hole, eliminating him from a three-way playoff.
His mettle was tested again at the first event of the FedExCup playoff, when playing in the last group on the final day of the Barclays Classic, he faded from contention.
“After all the things that have happened, to be back in this situation and finally get it done it’s an unbelievable feeling,” said Johnson, who will also be part of the US Ryder Cup squad that travels to Celtic Manor to take on Europe next month.
“Any time you can come out in the last group on Sunday and shoot a good round and win. It doesn’t get any better.”
Johnson was out of the gate quickly with a birdie on the opening hole to join Moore atop the leaderboard but it was Casey who made the power move by dropping five birdies to get to five-under through 12.
While Johnson held steady with an error free back nine that included just a single birdie at the 17th, Casey’s round unraveled with an ugly stretch of bogeys at 13, 14 and 15.
“Stumbled a little bit on the back nine,” said Casey. “I tried to make some birdies on the last couple of holes and fell short but congratulations to Dustin, he’s gone through a lot the last month or so.
“I played one shot worse than Dustin Johnson. That’s the way I look at it. It helps me sleep better at night.”
It was also a disappointing day for Tiger Woods as he saw his PGA Tour season come to an early end after failing to earn spot in the Tour Championship.
For the first time in 15-years, Woods finishes his PGA Tour season without claiming a single title but the world number one managed to exit on a positive note firing three birdies on a bogey-free back nine for a final round one-under 70.
The back-nine charge, however, was far too little, too late to earn Woods a place in the elite 30-man field for the lucrative season-ending Tour Championship in Atlanta.
His one-under 283 total left him in a tie for 15th, well out of the fifth place finish he needed to advance to the final round of the FedExCup.
“That’s just the way it is,” shrugged Woods. “I didn’t play well early in the year and I didn’t play well in the middle of the year.
