Johnson had three birdies on each side, salting the win away with birdies at Nos. 16 and 17 for a 72-hole total of 16-under 268.
Nick Watney had the best score in the fourth round with a 64 and wound up second at 14-under. Jim Furyk, who started the fourth round in the lead, was third at 13-under 271 after an even-par 71 Monday.
Most of the fourth round was delayed because of weather issues Sunday, leading to the unusual Monday finish and creating a short week for golf's best players. The Tour Championship begins Thursday in Atlanta.
The Top 30 players in the FedEx Cup standings are invited to the Tour Championship, with the first five in the standings -- Tiger Woods, Henrik Stenson, Adam Scott, Johnson and Matt Kuchar -- all able to win the FedEx Cup by winning the tournament.
Watney put early pressure on with his 64, which included three birdies to open his round and three straight over Nos. 12-14. That pulled him into a tie with Furyk and Johnson at 14-under.
Furyk, who moved to the top of the leaderboard with a second-round 59, staggered in with three bogeys in the span of Nos. 11-16 and slipped behind Johnson.
Johnson played very solidly and was bogey-free Monday. His lengthy putt at 16 gave him the sole lead and he dropped in a twice-breaking putt at 17 to secure the title.
It was Johnson's 10th PGA tournament title. He last won in July 2012 at the John Deere Classic, beating Watney by two strokes.