Johnson finished at 9-under 275, one better than Paul Casey (69), who fell from the top spot after bogeying three in a row from the 13th at the Cog Hill course in Lemont, Ill.
Johnson's BMW win may help to ease the sting of losing out in the final round of a pair of majors earlier this year, including the U.S. Open -- where he blew a three-shot lead -- and the PGA Championship, where he didn't realize he had been penalized for grounding his club.
Third-round leader Ryan Moore had a tough time Sunday, falling to a 2-over 73 and into a third-place tie with K.J.Choi (69), Kevin Na (70) and FedEx Cup points leader Matt Kuchar (72).
World No. 1 Tiger Woods shot a 1-under 70, good for a 15th-place tie at minus-1, while his playing partner, world No. 2 Phil Mickelson, carded a 4-under 67 to finish in a tie for eighth at 4-under 280.
Copyright 2010 United Press International