The four-time major, who had been upbeat about his title hopes despite missing the cut in last week’s Scottish Open, made a positive start with an opening birdie.
That was in stark contrast to the first-hole disaster that led to him missing the cut at Royal Portrush two years ago.
However, a run of three straight bogeys from the fifth had taken the wind out of his sails before rallying late on with two birdies in the last five holes to salvage a level-par 70.
“I’m probably proud,” admitted McIlroy, who raised a big cheer after knocking in a 15-footer for his 3 at the last.
“I got off to the perfect start and then made three bogeys in a row at the fifth, the sixth and the seventh.
“I then played the last 11 holes in two under which I thought was a good effort in those conditions. I hit some better shots coming in with some quality iron shots and I have to be encouraged by that.
“Mentally, it feels a bit better to birdie the last and I’m looking forward to getting out there on Friday.
“I said to Harry [Diamond, his caddie] walking to the eighth tee that we just had to try and get back to even. It was always the goal for the last few holes to get there.
“After that little wobble on the front nine, I set myself a target and to achieve it feels good.
“The difference between one over and level probably isn’t much at this stage, but it just makes you sleep that little better and makes me feel better about myself going into the second round.”
Playing in the match ahead, 2019 runner-up Tommy Fleetwood also signed off with a birdie as he carded a 67 - one of the best afternoon efforts.
“Tommy shot great and I thought anything under par would be good,” said McIlroy.
Darren Clarke, the 2011 winner here and McIlroy’s compatriot, opened with a 71 that contained two birdies and three bogeys.
Told he’d signed for that score despite only hitting nine greens in regulation, Clarke said: “Sometimes that's a funny stat whenever you're on links because if you're just off the fringe, which you're going to be an awful lot in links golf, you're going to use the putter. So my stats may have looked a lot better than they actually were.
“I actually hit the ball pretty nicely but I missed a couple of fairways by two inches and all of a sudden when you're in that you're trying to run the ball up or you're aiming away from flags to not make any stupid mistakes.
“But overall I was quite pleased with the way I actually hit the ball and the way I played.”
Asked how he felt being back at the Sandwich venue, he added: “It was wonderful. The Open Championship is always special to play in. It's even more special to win it and it's even more special to come back to the place where you've won it at before.”