The Northern Irishman carded a five-under-par 65 to sit just a shot off the first round clubhouse lead shared by Germany's Max Kieffer and Li Haotong of China.
"I'm not going to let three or four holes cloud my judgement in terms of how good I'm playing," 35-year-old McIlroy said.
McIlroy is competing for the first time since agonisingly missing out on a fifth major title at the US Open.
He held a two-shot lead with five holes to play at Pinehurst but bogeyed three of the last four before finishing a shot behind Bryson DeChambeau.
The devastating loss left McIlroy still looking to add to his last major which he won in 2014.
However, the 35-year-old insisted his game is in good shape ahead of the British Open at Royal Troon next week.
"I played well at the Canadian Open and at Memorial and the US PGA and Quail Hollow. My game has been in good shape and it was in good shape coming in here," said McIlroy.
"It's just a matter of going out there and focusing on the task at hand and not letting your mind wander too much. I felt like I did a good job of that today."
World number two McIlroy chipped in for an eagle on the third hole, his 12th of the day, and also recorded five birdies, but missed good chances on three of his last five holes.
"I hit a couple of shots over the first few holes that I haven't seen in practice over the last 10 days so it was a bit of a reminder that golf isn't as easy as sometimes I think it is," McIlroy said.
"But then I really started to feel good. Especially on the front nine, our back nine, I started to hit some good drives, some good iron shots."
Other star names taking part at the Renaissance Club in North Berwick include PGA Championship winner and 2022 Scottish Open champion Xander Schauffele, Jordan Spieth, Collin Morikawa and Ludvig Aberg.
