Also at six under were fellow American Brooks Koepka, who finished birdie-birdie, and veteran Englishman Greg Owen, who competed on the lower-tier Web.com Tour last season.
American Scott Brown, Australian Steven Alker, Briton Brian Davis and South African Richard Sterne lurk one back.
Fan favourite Phil Mickelson, a five-times major winner who will head to Chambers Bay searching for a first U.S. Open win after posting a record six runner-up spots in his national championship, returned a satisfying 68.
"I definitely putted well, thanks to the work I put in this morning," Palmer, whose most recent PGA Tour victory came at the 2010 Sony Open in Hawaii, told reporters after shooting the best round in the afternoon wave.
"I came out and spent about half an hour working on the speed of the greens and finally felt my comfort over the putter, the ball position I like.
"I had to get up and down a few times so it's nice when you make some tough pars to keep a round going like that."
Left-hander Mickelson was encouraged by what he described as "a good, solid round" that included four birdies on his outward nine followed by two bogeys after the turn.
"I got off to a good start, unfortunately I bogeyed the two par-threes on the front nine which, with short irons, you shouldn't really do," said Mickelson, who has not won on the PGA Tour since his 2013 British Open victory.
"The birdie putts on six and seven that I thought were going in just didn't quite fall so the second nine wasn't quite as low. But I hit a number of good shots today and more good shots than I did yesterday (in practice).
"If I can continue that trend, I feel like I am in the right direction, on the right path where I'm headed."
Fellow Ben Crane, who clinched last year's St. Jude Classic by a shot, launched his title defence with a 66.
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Steve Keating)
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