The 26-year-old Johnson will make his Ryder Cup debut next month
Final round leaderboard: -9 D Johnson (US) -8 P Casey (Eng) -6 R Moore (US), KJ Choi (Kor), K Na (US), M Kuchar (US) -5 R Goosen (SA)
Selected others: -4 P Mickelson (US) -2 I Poulter (Eng) -1 T Woods (US) level J Rose (Eng) +3 L Donald (Eng), R McIlroy (NI) +8 M Laird (Sco) +9 B Davis (Eng)
Paul Casey struggled over the closing holes of his final round at the BMW Championship in Illinois to finish a shot behind winner Dustin Johnson.
Casey dropped shots at the 13th, 14th and 15th and then saw Johnson birdie the 17th to edge ahead for victory.
Johnson, who finished on nine under, had come close to major wins at both the US Open and the PGA Championship.
But Tiger Woods, who ended on one under, will miss the play-off finale at the Tour Championship in Atlanta.
The world number one, who earlier this week was named as one of US captain Corey Pavin's wildcards for next month's Ryder Cup, did not do enough to get into the top 30 in the FedEx Cup standings and qualify for the season-ending tournament which starts on 23 September.
Casey, who was left out of Europe's Ryder Cup team, was three clear after a superb first 12 holes before losing his grip on the title.
I've had some unfortunate situations on Sundays this year, so this feels great
"I stumbled a little bit on the back nine so that was disappointing, but congratulations to Dustin Johnson - he's gone through a lot," said Casey, who moves up from ninth to seventh in the world rankings.
"It was myself against the golf course and I played it one shot worse than Dustin Johnson. That's the way I look at it - makes me sleep at night."
Johnson, who moves up to a career-high 12th in the world, led by three at the US Open in June but then shot a final-round 82 to miss out on a first major win.
And then in last month's USPGA he was one in front with one to play, bogeyed and was controversially denied a play-off when given a two-stroke penalty for grounding his club in what he thought was "a piece of dirt", but was deemed a bunker.
Casey (left) squandered his lead while Poulter failed to mount a challenge
The 26-year-old, who will make his Ryder Cup debut next month, is now second in the FedEx points race and could scoop the $6.5m bonus if he wins the final leg.
"I've had some unfortunate situations on Sundays this year, so this feels great," he said.
"I felt like I really stayed in the moment. I didn't get ahead of myself. I tried not to watch the leaderboard too much. I really did a good job of just being patient."
American Ryan Moore, who held a one-shot lead going into the final round, finished tied third alongside FedEx Cup leader Matt Kuchar, Kevin Na and South Korean KJ Choi.
European Ryder Cup team-mates Rory McIlroy and Ian Poulter failed to qualify for the season-ending Tour Championship, with Poulter dropping out of contention after taking a triple bogey eight on the 11th.
Luke Donald played his last nine holes in 42 to drop to three over and is seventh in the standings, with Scotland's Martin Laird ninth and Justin Rose 13th.