The Australian closed out his tournament with a two-under-par 69 to finish at 22-under 262 at Conway Farms, six clear of American rookie Daniel Berger (69) in the third of the PGA Tour's four FedExCup playoff events.
American Scott Piercy (70) was third at 15-under with Rickie Fowler (69), Rory McIlroy of Northern Ireland (70) and JB Holmes (68) sharing fourth at 14-under.
It is the seventh tour win for Day, the fifth of the season after winning the Farmers Insurance Open, the Canadian Open, his maiden major at the PGA Championship and the opening playoff event, The Barclays, prior to this triumph.
He led by as many as seven and by no less than four in the final round and faced no serious challenge as he played conservatively throughout.
Day becomes the third Australian to reach world No 1, following Greg Norman and Adam Scott.
He now heads to the 30-man, season-ending Tour Championship in top spot on the FedExCup points list, ensuring a win at East Lake Golf Club would see him claim a $US10 million bonus.
Masters and US Open champion Jordan Spieth, Fowler, Henrik Stenson of Sweden and Bubba Watson will also control their fate next week in Atlanta by holding spots in the top five of the points list.
