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.
"The way I feel about my game, confidence level has been the highest I've ever had in my entire life," Day said.
"Its been an amazing kind of run for me, and now it just tops it off even better to get to number one in the world.
"I felt a lot of pressure over the last two days, especially today just knowing I had a six shot lead.
"Yesterday and today were probably the toughest rounds I've ever had to play in my entire life."
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.
