Playing in the final group for the fourth time since June, Johnson blasted a tee shot over the trees and into the fairway on the 17th hole at Cog Hill to set up a tap-in birdie and the outright lead. He closed with a 2-under 69 and a one-shot victory over England's Paul Casey.
It was the second victory this year for Johnson, a 26-year-old American who has quickly emerged as one of golf's rising stars. And it helped ease the sting of lost chances in three other tournaments, including two majors.
He squandered a three-shot lead at the U.S. Open. He made up a three-shot deficit at the PGA Championship, only to lose a one-shot lead on the final hole when he didn't realize he was in a bunker and grounded his club.
''To finally get it done, especially after all the things I've gone through this summer ... it can't feel any better,'' said Johnson, who finished at 9-under 275 and won $1.35 million. ''I played really good golf today.''
Johnson was flawless to the very end on the back nine Sunday.
He moved to No. 2 in the FedEx Cup standings, giving him a clear shot at the $10 million bonus in two weeks at the Tour Championship.
One player he won't have to beat at East Lake is Tiger Woods. The world's No. 1 player sputtered at the start and shot 70 to tie for 15th, not nearly enough to move into the top 30 in the standings and advance to the FedEx Cup finale.
It's the first time as a pro that Woods hasn't been eligible for a tournament.
''That's just the way it is,'' Woods said. ''I didn't play well early in the year, and I didn't play well in the middle of the year.''
His next event will be the Ryder Cup in two weeks, and he won't be the only Ryder Cup player with two weeks off. Woods and two other Ryder Cup picks, Stewart Cink and Rickie Fowler, also failed to qualify for the Tour Championship.
Woods played with Phil Mickelson for the first time all year, and Lefty buried him. Mickelson closed with a 67 and tied for eighth, his first top 10 since the U.S. Open.
It was the 26th time the world's best two players have been in the same group, and the record now stands at 11-11-4.
Casey was left off the Ryder Cup team despite being No. 9 in the world when European captain Colin Montgomerie made his three picks. He had a great chance to make Montgomerie look foolish, building a three-shot lead on the back nine, only to throw it away with three straight bogeys. Casey had three chances from the fairway coming in, but didn't give himself a birdie opportunity inside 25 feet.
''It was myself against the golf course, 72 holes, and I played it one shot worse than Dustin Johnson,'' Casey said.
With the 70-man field at Cog Hill narrowed to the final 30, the BMW Championship offered plenty of drama away from the leaders, and Matt Kuchar had the greatest influence.
Kuchar tied for third and will be the No. 1 seed in two weeks at East Lake. He was at 7 under and needed a birdie for any hope of getting into a sudden-death playoff, but he ran his birdie putt about 3 1/2 feet by the hole.
He missed that putt, which dropped him from a two-way tie for third into a three-way tie for fourth. That allowed K.J. Choi, who tied for fourth, to get into the top 30, knocking out Bill Haas.
Ryan Moore, the 54-hole leader, was alone in third place when he sent his second shot over the 18th green. He smartly played away from the pin to avoid chipping into the water and took his bogey, which kept him in the top 30. If Moore had made double bogey, he would have fallen out of the Tour Championship.
Charlie Wi was poised to make the Tour Championship for the first time until he bogeyed the last two holes for a 74. Wi needed only a par to finish in the top 30, but drove into the trees on the 18th and missed a 12-foot par putt.
The 30th and final spot went to Bo Van Pelt.
