The world number two was looking to bounce back from narrowly missing out on a first major title for 10 years at the US Open last month.
But McIlroy's dreams of a second Claret Jug turned into a nightmare in the wind and rain on Scotland's west coast.
Things began to go wrong for the Northern Irishman at the par-three eighth where he found a green-side bunker with his tee shot and then rolled back into the sand with his second to make a double bogey.
He then fired out of bounds to post another double bogey at 11 and also dropped shots at the 10th, 15th and 18th.
"Just one of those days where I just didn't adapt well enough to the conditions," said McIlroy.
"All I need to focus on is tomorrow and try to make the cut. That's all I can focus on."
McIlroy was far from the only big name to struggle with the combination of tough conditions and a challenging course.
US Open champion Bryson DeChambeau needed an eagle on the 17th to post a five-over 76, while world number four Ludvig Aberg hit 75.
McIlroy explained how he had been caught out by cross-winds coming from a different direction to what he had faced in practice earlier in the week.
He last missed the cut at the British Open on home soil at Portrush in 2019, when a six-under par second round could not undo the damage of an eight-over 79 on day one.
"It was more like the conditions got the better of me," added McIlroy.
"I need to go out there and play better and try to shoot something under par and at least be here for the weekend, if not try to put myself up the leaderboard a bit more and feel like I have half a chance."
