Day and Noren had to return after grinding through five playoff holes Sunday before it got too dark to continue. They played the par-5 18th on Monday morning, their fourth time visiting the finishing hole in the playoff.
Tiger Woods, who had to rally on his back nine on Friday to make the cut, finished his up-and-down final round early on Sunday by shooting even par. Woods, who consistently had huge galleries and TV cameras following him throughout the weekend, finished the tournament at 3 under, 7 strokes off the lead.
On Monday, Day teed off at 8:04 a.m. and sank the winning putt 13 minutes later, his fourth birdie on the par-5 hole in the playoff. It was Day's 11th PGA Tour win of his career, and first since the 2016 Players Championship. He moved to No. 9 in the FedExCup standings.
His victory was accompanied only by the sounds of camera shutters and cheers from Day's family members. Spectators weren't allowed in because organizers said they didn't have time to arrange for security.
Noren tried an aggressive second shot from the first cut, but his ball landed short of the green and rolled into Devlin's Billabong, a pond that protects the hole. The Swede moved to 34th in the FedExCup standings with his career-best TOUR finish.
Day, meanwhile, hit his second shot out of the rough and over some trees onto the fairway before hitting a beautiful approach shot that set up the winning putt.
Noren bogeyed the hole.
"It's been a long time coming," said Day.
He had a dreadful 2017, which included his mother recovering from lung cancer and a game that lost some discipline and focus.
Noren, 35, was trying for his first PGA TOUR win. The Swede, who played at Oklahoma State, has nine victories on the European Tour, including four in 2016. He is No. 19 in the world ranking.
It was the longest playoff in the tournament's 67-year history.
Day and Noren matched each other with birdie, birdie, par, par and birdie through five playoff holes in the twilight Sunday on the blufftop course overlooking the Pacific Ocean.
Ryan Palmer was eliminated on the first extra hole with a par.