Morikawa is now only the second player after Tiger Woods to win both a major and a WGC title before turning 25.
A week after missing the cut in Minnesota in the 3M Open, Brooks Koepka had nine birdies and a bogey on a windy day at TPC Southwind in the World Golf Championship event.
The decision means Tiger Woods, who is one win shy of a record-breaking 83 PGA Tour victories, will go into the year’s first major having played just one event since the PGA Tour returned in June.
McIlroy won for the fourth time this year. It was his third World Golf Championships title, and his first since the Match Play at Harding Park in 2015.
McIlroy finished with his third straight five-under par 67 of the week to move to -15 at the Sheshan International Golf Club
The $10.25 million Shanghai event, with a winner's prize of $1.7 million, is the third and final leg of the PGA Tour's Asian swing, after the Zozo Championship and the CJ Cup in South Korea the week before.
It was the first time Koepka and McIlroy had been paired in a final round, raising hopes of a day-long duel. But McIlroy missed a short birdie putt at the third hole from inside four feet and never recovered.
Sharma believes he is close to rediscovering the form that led him to two wins on the Asian Tour and is capable of following the footsteps of Hideki Matsuyama, who won the title in 2016.
WGC Invitational: Shubhankar Sharma aims to perform well at Firestone Country Club and meet childhood idol Tiger Woods
Sharma will make his debut at the Bridgestone Invitational headlined by title holder Hideki Matsuyama, Dustin Johnson, Francesco Molinari, and 14-time major winner Tiger Woods.
Shubhankar Sharma believes a golfer from India will soon win a Major, but sport needs more icons to take it to masses
Shubhankar Sharma said Indian golfers have made good progress in recent years and the day is not far when an Indian will win a Major championship
WGC Mexico Championship: Phil Mickelson wins first title in five years, India's Shubhankar Sharma finishes ninth
Mickelson had sounded a confident note when he finished the third round two shots off the 54-hole lead of India's Shubhankar Sharma.
Lifted by two wins in as many months, India's latest golfing star Shubhankar Sharma has earned his maiden spot in the World Golf Championships (WGC) to be held at the Club de Golf Chapultepec next month
Tiger Woods will return to competitive golf at the Hero World Challenge after nine-month injury layoff
Woods is included in an 18-man field which features many of world golf's elite, including World No 1 Dustin Johnson and World No 2 Jordan Spieth.
Tiger Woods has dropped outside the top 1,000 players in the world for the first time in his professional career according to the latest rankings on Monday.
Anirban Lahiri will seek to fast-track his progress on the PGA Tour when he tees up at the high-profile World Golf Championships (WGC), starting Thursday.
Anirban Lahiri never trailed in the match and beat his more favoured opponent 4&2.
All set to make his debut in the prestigious WGC-HSBC Champions next week after a stellar run this season, in-form Indian golfer Anriban Lahiri today said the values he grew up with have played a huge role in his meteoric rise.
Woods put himself in prime position to seal a record eighth victory in the elite World Golf Championships (WGC) event at Firestone Country Club, shooting a two-under-par 68 on Saturday to post a 15-under total of 195.
Scott fired a sizzling eight-under-par 62 to take control in the elite World Golf Championships. But most eyes at Firestone Country Club were focused on seven-times champion Woods who carded a 68 on his long-awaited PGA Tour return.