Poulter, who was ranked outside the world’s top 200 just 15 months ago, carded a one-under-par 69 to join American Scott Piercy at the top of the leaderboard at Shinnecock Hills.
But McIlroy’s hopes of a second US Open title and first major since 2014 were blown away as he slumped to a 10-over-par 80, his worst score in the US Open, taking his combined total since winning with a tournament-record score of 16 under in 2011 to 53 over.
Playing partners Jordan Spieth and Phil Mickelson fared little better with rounds of 78 and 77 respectively, while former British Amateur champion Scott Gregory slumped to an unfortunate 92, the first score in the 90s in this event since 2002.
Poulter missed the cut on his US Open debut at Shinnecock in 2004 and has yet to record a top-10 finish, while his 69 is the first time he has broken 70 in the opening round.
“I did not enjoy it at all in 2004 and through most of the US Opens it feels like you are pulling teeth,” said Poulter, who claimed his first victory since 2012 in the Houston Open earlier this season.
“It’s supposed to be tough but this week I’ve changed my mindset. I’m here to enjoy my golf, play freely and just go and play. It was brutal out there and I’m glad they have widened the fairways otherwise I don’t know what the scores would have been.”
McIlroy had been bullish about his prospects after a lengthy spell of preparation at Shinnecock and other courses on Long Island, but after missing from seven feet for birdie on the 10th, his opening hole, he dropped six shots in the next four holes.
A birdie on the 15th briefly stopped the rot, but McIlroy bogeyed the 16th and 18th to reach the turn in 42 and then ran up a double bogey on the first after a wayward tee shot.
It is the first time the former world number one has carded three double bogeys in a round in the majors and although he birdied the fifth and sixth, further shots were squandered on the seventh and ninth.
That left McIlroy needing to emulate joint-leader Piercy’s reversal in fortunes to have a chance of making the cut, the American having walked off the course in frustration at the state of his game on Wednesday.
“I was shanking it and lost like five balls in the first four holes. I’m like ‘I’m outta here”’ Piercy explained. “I needed some time away so we went back to the house, ordered some pizza and I actually went back on my Instagram.
“I looked at some swings that I posted, positions that I was in, saw some drills I was doing and then just ran from there.”
There was plenty of sympathy for the unfortunate Gregory after his round but he said he would always have one happy memory from Shinnecock Hills.
“I didn’t get it in play off the tee, which is obviously a big issue around here,” said Gregory, who came through international qualifying at Walton Heath to book his place in the field. “Any rough you go in you can’t really attack pins and get on the greens.”
Gregory secured a berth in the Masters and US Open last year after winning the British Amateur title in 2016 and also received an invite to the Memorial Tournament, where he had hoped to meet Tiger Woods.
However, with Woods not competing at Muirfield Village due to injury, the 23-year-old from Portsmouth had to wait until this week to secure a treasured memento.
“I’ve been waiting for that picture for about 15 years,” Gregory said. “It was great to finally meet him. I think everyone in the golfing world is happy to see him back.
“It’s weird, you can feel when he is about. If you’re on the putting green and you haven’t seen him, you can still hear that he’s around.”