The defending champion recovered from a triple-bogey seven on the fifth, which ended his hopes for a second straight PGA Championship on Sunday.
The 24-year-old finished the tournament at 3 under, one shot ahead of fellow Northern Irishman Graeme McDowell, who carded an excellent closing 66.
McIlroy hit back with birdies on the ninth, 10th and 13th, making just one bogey the rest of the day.
"I played the best golf of the week today," said the Holywood golfer, who finished seven behind champion Jason Dufner.
"I hit some really good drives and really good iron shots. Didn't quite putt as well as I did the first three days, but really, really happy with my game going into the next few weeks."
It's definitely a step in the right direction.
"It's close," McIlroy said of his game.
"I saw a lot of great signs out there today. Hopefully I can just bring that through to the next few weeks and have a strong finish to the season."
"I'm just disappointed because I felt like if I would have capitalised on the way I played, (the outcome) would have been much better," McIlroy said of his final round.
"I've got four big weeks coming up and want to finish the PGA TOUR season well, and I have some events in Asia, so I could still salvage a bit of the season," he said. "I saw a lot of great signs out there today."
Meanwhile Darren Clarke struggled to an 80 and finished at the bottom of the weekend leaderboard.
American Dufner claimed his first major title at Oak Hill.
The 36-year-old, who equaled the lowest score in major history with a second-round 63, carded a closing 68 to finish 10 under par.