He pronounced himself ready and confident on the eve of the championship, but the PGA Tour leader in numerous driving stats was too often forced to slash errant tee shots out of the tall fescue outside Erin Hills' generous fairways.
He's 13 strokes behind Rickie Fowler, who is now one stroke ahead with players still on the course.
But that didn't happen and the 117th US Open became the first major not to include Mickelson or Tiger Woods since the 1994 Masters.
Other contenders who enjoyed the day one conditions were Americans Brian Harmon and Brooks Koepka as well as Englishman Tommy Fleetwood who all shot five-under 67s. They combined for just two birdies; the third member, Martin Kaymer, had four birdies in shooting the group's low round, an even-par 72.
Jordan Spieth (+1), Adam Hadwin (-4), Lee Westwood (-3), Adam Scott (E) and Justin Rose (E) are all on the board at 30/1. They are 150/1 and 300/1 respectively to win.
"Once you get through six or seven misses the hole starts to look smaller and smaller". "I don't know that I've ever played a U.S. Open where I've had that much enjoyment".
McIlroy won the U.S. Open at Congressional in 2011 in an eight-shot runaway, one of his four major championship titles.
"I felt very comfortable out there", said the Clara man after his one-under-par 71, secured with a birdie on the last.
The worst two came toward the end of the round, on the 15th and 17th holes, when errant tee shots led to double bogeys on both occasions. After missing the cut in the Memorial Tournament, the world number one has now made just three birdies in his last 54 holes. I don't boast it as a truly great round of golf, even though it's really low because I still made errors.
"I'd like to think we learned a little bit and thank him for that this morning".
No surprises that the easiest hole was a par five, but at 632 yards the 18th was not supposed to be such a pushover.
"You can not play this golf course if you're not in position off the tee, and I wasn't in position", he said, according to the AP.
Six times a runner-up at the U.S. Open, Mickelson skipped another chance at completing a career grand slam when it became obvious Mother Nature was not going to cooperate and delay the start of the opening round.
"I've got some improving to do, but I don't mind that".
"It was nice. You don't get many rounds at the US Open that are stress-free. Maybe they'll move up the tees.' The players get it and I had a good time". I knew I needed to drive it well and from there I continued to swing well, hit good shots and roll a couple in. "It's the kind of golf course that if you're on point and playing well and you're in the fairways, you really feel like you can be aggressive and attack and make some birdies".