Sam Saunders took the first-round lead in the Genesis Open, a tournament his late grandfather Arnold Palmer won three times when it was known as the Los Angeles Open.
Percy, who only just regained his PGA Tour card via the Web.Com Tour Finals past year, raced to the turn in 31 and although the momentum slowed over the closing nine holes his birdie at the last was a nice way to finish. He finished the day with six birdies and one bogey.
Former world No1's Adam Scott and Luke Donald posted matching 68s, with English pair Justin Rose and Paul Casey a further shot back.
"(It's) kind of a little bit of a bonus", Saunders said of completing the round, adding that he was "mentally prepared" for the possibility that weather could prevent him from playing at all on Friday. "Nobody can. Nobody's going to compete".
Mickelson has won here twice and lost twice in playoffs.
He then treaded water until the par-five 17th, where a stellar approach settled two feet from the flag and Scott tapped in for another birdie. He regained a tie for first with DJ and two others among the morning wave with his fifth birdie of the day, at No. 7, but had to settle for a share of second, at 4-under 67, after a disappointing bogey at the last.
Joining Johnson at five under par was J.T. Poston, a Monday qualifier for the event who holed out from the fairway to eagle the par-five first hole for a rousing start on a day when fog delayed play for about an hour.
The three-time major victor would also be entitled to feel a little disappointed after he failed to birdie two of the three par fives.
Padraig Harrington is in the chasing pack with Billy Hurley III, Phil Mickelson, Pat Perez, Branden Grace and Kevin Na.