Mickelson skipped practice rounds at the course on Tuesday and Wednesday to fly back to California and attend his daughter Amanda's eighth-grade graduation ceremony, then flew back in time to start the Open on Thursday.
The airplane landed at Philadelphia at 04:15 (10:15 CAT) in the morning and the 42-year-old left-hander reached the course 87 minutes later, well ahead of his 07:11 (13:11 CAT) tee time.
Seeking a first US Open title after finishing runner-up a record five times, Mickelson then set about tackling Merion's East course, hosting the US Open for the first time in 32 years.
He opened with a bogey at the 11th, but was back on level par before a storm system dumped torrential rain on the course and sent the morning starters scurrying for cover after just 111 minutes of play.
When the round resumed 3 hours 32 minutes later, Mickelson, a four-time major champion, used his supreme short-game skills to added birdies at the first (his ninth), seventh and ninth holes, the last where his putt went in from 30 feet.
His three-under 67 was his lowest opening round in the US Open since 1999.
It also gave him the clubhouse lead and the luxury of putting his feet up and watching the afternoon starters try to match him, knowing that due to the delay they would be unable to complete their rounds by nightfall.
Also in the clubhouse early with good scores were Belgian Ryder Cup hero Nicolas Colsaerts, who had a 69, while Americans Bubba Watson, Dustin Johnson and Steve Stricker all carded one over 71s.
Out on the course, Jerry Kelly was at two under through 16 with amateurs Pan Cheng-Tsung of Taiwan and Kevin Phelan of Ireland, 2003 Masters champion Mike Weir of Canada and South Africa's George Coetzee all at one under with a handful of holes to go.
The South African trio of Coetzee, Charl Schwartzel, and Tim Clark all shared the early lead at one stage during the day, as Ian Poulter made the early running, falling off again as his round progressed.
Tournament favourite Tiger Woods had been due off at 13:14 pm (19:14 CAT), going out with the two players next to him in the global rankings -- Rory McIlroy and Adam Scott, but the delay meant his start time was put back until 16:48 (22:48 CAT).
That meant they and the 25 other afternoon groupings in the 156-strong field would not be able to complete their opening rounds, even if a secondary belt of bad weather, forecast for late in the afternoon, failed to materialize.
Woods, whose last major win came at the US Open at Torrey Pines in 2008 when he won an 18-hole playoff against Rocco Mediate on what turned out to be a broken leg, is back near his best with four tournament wins this year already.
But to seal his comeback he needs to win a 15th major title and move to within three of the all-time record of 18 set by Jack Nicklaus when he won the 1986 Masters.
Woods, at 37, is nine years younger than Nicklaus was when he last won at Augusta National.
Playing partner McIlroy is still waiting for his 2013 season to catch fire after an equipment change that has proved more problematic than he expected.
But he has a record of rebounding from poor form at US Open time and has sought inspiration in Philadelphia by visiting "The Rocky Steps" made famous by Sylvester Stallone in his film portrayal of boxer Rocky Balboa.
The field includes 10 former US Open winners and a host of major champions from six continents.
Scores on Thursday during the storm-interrupted first round of the 113th US Open golf championship at 6,996-yard, par-70 Merion Golf Club (78 players unable to finish due to darkness, will complete first round on Friday):
67 - Phil Mickelson (USA)
69 - Nicolas Colsaerts (BEL)
70 - Charl Schwartzel (RSA), Tim Clark (RSA), Jerry Kelly (USA), Rickie Fowler (USA), Jason Day (AUS)
71 - Bubba Watson (USA), Dustin Johnson (USA), Steve Stricker (USA), Justin Rose (ENG), George Coetzee (RSA), Charley Hoffman (USA), John Huh (USA), Ian Poulter (ENG), Kevin Phelan (IRL)
72 - Hunter Mahan (USA), Stewart Cink (USA), Shawn Stefani (USA), Nicholas Thomson (USA), Mike Weir (CAN), Pan Cheng-Tsung (TPE)
73 - Nick Watney (USA), Paul Casey (ENG), Marcel Siem (GER), Sergio Garcia (ESP), Padraig Harrington (IRL), Michael Kim (USA), Chris Doak (SCO), Jaco Van Zyl (RSA), Kevin Sutherland (USA), Ryan Nelson (USA)
74 - Peter Hanson (SWE), Lucas Glover (USA), Matt Kuchar (USA), Brandt Snedeker (USA), David Lingmerth (SWE), Martin Laird (SCO), Jason Dufner (USA), Henrik Stenson (SWE), Simon Khan (ENG), Jay Don Blake (USA), Brandt Jobe (USA), Randall Hutchison (USA)
75 - Cliff Kresge (USA), Aaron Baddeley (AUS), Louis Oosthuizen (RSA), Boo Weekley (USA), Matteo Manassero (ITA), Ryan Palmer (USA), Douglas Labelle (USA), Matt Weibring (USA), John Hahn (USA), Mackenzie Hughes CAN)
76 - Ryan Yip (CAN), John Parry (ENG), Ted Potter (USA), Michael Campbell (NZL), Wil Collins (USA), Harold Varner (USA)
77 - Bill Haas (USA), Keegan Bradley (USA), Rory Sabbatini (RSA)
78 - Rikard Karlsberg (SWE), Yui Ueda (JPN), David Hearn (CAN), Geoffrey Sisk (USA)
79 - Ryan Moore (USA)
80 - Roger Tambellini (USA), Robert Garrigus (USA)
81 - Andre Svoboda (USA), Cory McElyea (USA)