Willett held a two-shot lead on the 18th tee after making a birdie on the 17th, but pushed his tee shot slightly into the bank of a creek that divides the 18th fairway, and then hit a brave second shot from the rocks before making par.
England’s Matt Wallace (68) and American Patrick Reed (70) were joint runners-up at 16-under 272.
Starting the day at 14-under, Willett edged ahead of the field with an eagle on the second and never gave up the advantage. Reed, the joint overnight leader, could not find a birdie until the 10th hole and made two bogeys on the front nine to take the pressure off his playing partner.
Bogeys on the 10th and 12th holes opened the door for his rivals, but he steadied the nerves and bounced back with three birdies in four holes starting on the 14th.
“We got off to a flying start and everything seemed rosy in the camp. Then obviously there was a blip there down 10 and 12. I am very proud of how I handled that and how I handled myself emotionally after that,” said Willett, who has battled a long-running back injury.
“Winning’s a rarity on tour, really. I’m pleased to have won the tournaments that I’ve won over the last few years. But this, coming back after everything that’s happened, is going to go down in the history books for myself as one of the most pleasing.”
Molinari finished his round and even though it was evident that Fleetwood wasn’t going to win the tournament, he spoke to the media only after Dylan Frittelli posted a score better than Fleetwood’s.
“It means a lot. You look at the players who have won the Race to Dubai or the Order of Merit before that, and obviously it’s only really the top players that have done it,” said the 36-year-old Italian.
“I never thought something like this would happen to me to be honest, and now it’s going to be a challenge to reset before next year and work as hard as I did the past winter and try to reproduce the same golf.”
India’s Shubhankar Sharma, a two-time champion this season, won the Rookie of the Year honor by finishing 28th in the Race to Dubai.