He only played like a well-worn veteran.
McIlroy, a golf prodigy from Northern Ireland, will celebrate his 21st birthday on Tuesday, but Sunday he played a final round for the ages. Beginning the afternoon four shots out of the lead, he finished four strokes in front of runner-up Phil Mickelson. He made eight birdies, an eagle and no bogeys.
"It started on the range," he said. "I absolutely flushed it on the range and knew. I was excited to go out there and play."
It showed. To claim his first PGA Tour victory and become the PGA Tour's youngest winner since Tiger Woods was a rookie in 1996, McIlroy carded a back-nine 30, eagling the par-5 15th after a 6-iron from 206 yards to inside four feet, then going birdie, par, birdie. He closed the tournament with six straight 3s on his scorecard and seven out of the last eight.
On the final hole, from 42 feet away and trying only to two-putt for a par, he rolled in the birdie.
How's that for a finish?
Also See: New Major Tour Record Set in Japan
"He could have birdied the last three," said proud countryman David Feherty, the CBS commentator. "You are supposed to soil yourself on the last three, especially here -- and you are 20 years old.
"That was strong."
It didn't seem to matter to McIlroy that Mickelson, the three-time and reigning Masters winner, and two-time major champion Angel Cabrera were giving chase, both carding rounds of 68.
He never looked back.
"I was six holes into the round thinking that 68 was kind of the number, that 11 (under) was the number I need to try to get to," Mickelson said after finishing 11 under. "I've got to congratulate Rory on a tremendous round. Thirty on the back nine on this course is just fabulous."
The victory is a confirmation of what golf has long been anticipating.
A year ago, at just 19, McIlroy won the European Tour's Dubai Desert Classic and later pulled off a 20th-place finish at the Masters. After turning 20, he tied for 10th at the U.S. Open and tied for third in the PGA Championship.
It sent expectations for this season soaring. But after finishing third in the European Tour's Abu Dhabi Championship and fifth in the Dubai Desert Classic early in the year, McIlroy's production fell drastically when he arrived in the U.S.
Nursing a sore back, he came to Quail Hollow having yet to be a tournament factor, including, most recently, two consecutive missed cuts at Houston and the Masters.
At that point, he was mulling the idea of putting the clubs away for a break and skipping the Quail Hollow event.
"Yeah, that would have been a good idea," McIlroy said with a laugh. "But I got home and took a few days off. I just said to myself, 'Look, there's no point in feeling sorry for yourself here. You're not playing great, you need to go and work.' "
All the same, by early indications this week, he was on the way to another uninspiring performance. McIlroy finished two rounds at 1-over par to make the cut on the number and began Saturday by bogeying his opening hole.
Then everything changed. From there McIlroy birdied nine of his remaining 17 holes on Saturday to shoot 66 and climb within four shots of the 54-hole lead.
He returned Sunday to play what, when the season finally comes to an end, will likely be judged as one of the year's best rounds.
"Call it in the zone or call it whatever you want to, but I was just seeing my shots," McIlroy said. "I was hitting them. I was getting it up-and-down. I was seeing putts go in. I was reading the lines.
"It's pretty cool when it happens, because it doesn't happen often. But when it does, you sort of have to make the most of it."
Mission accomplished. When it was over, McIlroy had played the tournament's final 36 holes 16 under.
"This is huge," McIlroy's caddie, J.P. Fitzgerald said. "To win over here, especially after the last few weeks ... He's worked hard."
The win will be a popular one. McIlroy's shaggy haircut and respectful demeanor belies the celebrity status built on expectations. You could figure that out by watching the tour pros who went out of their way to offer congratulations: Lee Westwood, Padraig Harrington and Jim Furyk among them.
"Yeah, I really appreciated it," McIlroy said. "You know, it's nice that the guys want me to do well, and to have a good bunch of people around you and a good bunch of friends out here definitely helps because when you're not playing well, it can be a lonely place at times."
Now will come another compliment.
As the youngest PGA Tour winner since Woods claimed the 1996 Las Vegas Invitational at 20 years, 9 months, 6 days, a comparison is inevitable.
"He was just going further and further ahead," Feherty said. "That doesn't remind you of somebody?"