Mickelson had flown home to California on Monday after torrential rain saw the course closed in mid-afternoon, and then attended his daughter's eighth-grade graduation on Wednesday evening before flying back in time for his 7:11am tee time on Thursday morning.
He landed in Philadelphia at 3:30am in his private jet and was at the course at 5:40am, only to start his round from the 11th with a three-putt bogey.
However, he birdied the 13th and was level par when a thunderstorm and torrential rain caused play to be suspended for more than three and a half hours, and when it did resume the four-time major winner picked up birdies at the first, seventh and ninth to card an opening three-under-par 67.
That gave the left-hander, who turns 43 on Sunday, the clubhouse lead, although England's Luke Donald was four under par after 13 holes when play was suspended for the day.
Playing partner Lee Westwood, seeking his first major title at the 61st attempt, had been three under after 11 holes, but saw his third to the par-four 12th hit the wicker basket which is used instead of a standard flag and bounce back off the green, leading to a double-bogey six.
Masters champion Adam Scott was three under after 11 holes with Rory McIlroy level and Tiger Woods - the world's top three playing in the same group - two over. Play was scheduled to resume at 7:15am on Friday.
"If I'm able, and I believe I will, to ultimately win a US Open, I would say that it's great," Mickelson said of his relationship with the tournament.
"Because I will have had let's say a win and five seconds. But if I never get that win, then it would be a bit heart-breaking."
Mickelson's daughter Amanda was born the day after his first second place in the US Open at Pinehurst in 1999, where he wore a beeper on the course during the final round in case his wife went into labour.
He revealed she knew "a little bit" about her part in the drama and that she had quoted Ron Burgundy from the film Anchorman in her speech at the graduation ceremony.
"She told me stay, it's the US Open, I know how much you care about it," said Mickelson, who has five wedges but no driver in his bag this week.
"And I told her that I want to be there. I don't want to miss that. I don't want to miss her speech, I don't want to miss her graduation. She spent nine years at that school, she's worked very hard and I'm very proud of her.
"She did a great job and she even quoted Ron Burgundy, so it was funny. The ceremony was at 6pm. I got on the plane at 8pm, landed 3:30am. I had a couple of hours sleep on the plane, an hour before we teed off and an hour during the break. I feel great.
"This is not that out of the ordinary, I do this about six to 10 times a year where I fly back east (on the) red eye, play some outing and then come home.
"When I was here (Merion) the week before I was able to do all the work I needed, the last part was getting my game sharp so being able to do that in nice weather on a good practice facility was advantageous."
Belgium's Nicolas Colsaerts was the only other player to complete a sub-par round with a 69 to finish two behind Mickelson.
Colsaerts, who has struggled to hit top form on the PGA Tour this season, missing the cut in his last two events and also at the Masters, said: "I don't think I've really peaked anytime during the season. It's not very far off, but when you get to play courses that are pretty demanding anything can happen, just as is proven today, really.
"I'm relieved (to get the round finished). We know that we are probably going to finish the second round on Saturday morning. Whatever side of the draw you're on, you're going to have to deal with some issues."
Ian Poulter had made a brilliant start with birdies on his first three holes and was still one under with five to play before having to settle for a 71 to finish alongside good friend Justin Rose.
That was just two shots better off than Ryder Cup team-mate Sergio Garcia, who was occasionally heckled as he battled back superbly after a nightmare spell either side of the first suspension.
The Spaniard had been warned by Colin Montgomerie that he could be booed by fans in the wake of his "fried chicken" row with Tiger Woods, with 'pollo frito' - the Spanish for fried chicken - one of the things shouted, but there were also plenty of cries of 'Go get 'em Sergio' and Garcia wisely played down any incidents.
"There were a couple (of wisecracks) but I thought the people were very nice. Almost all of them were behind me and that was nice," said Garcia, who was seven over through eight holes after hitting consecutive drives out of bounds on the 14th and 15th, but eventually returned a 73 after three birdies and an eagle.