Chalmers rolled in eight birdies, including four of his final six holes, and had just one bogey at La Paloma Country Club for a two-shot edge over fellow countryman Rod Pampling, Canada's Stephen Ames and South Africa's Ernie Els.
Chalmers, 51, is in his second season on the tour and is searching for his first victory.
"I've got a lot of good processes in place and just hit good shots and we'll see what the score it is," Chalmers said. "... I'm just going to try to keep hitting good shots and see the putts fall in and we'll see what happens."
He was particularly pleased with how he closed his first round.
"Just a really quality iron shot on the 17th," Chalmers said. "I hit a seven-iron that I actually faded in there about eight feet right of the hole, really tricky read and I managed to make that."
"Then I had a lob wedge into 18 and I hit that six feet left of the hole and made that. That was a makeable putt for me, left-right putt. It's just a combination of some good shots, and they're downwind today, so a little more friendly."
Els, the World Golf Hall of Famer, had a busy scorecard with six birdies, three bogeys and a chip-in eagle at the par-5 16th. Ames and Pampling went bogey-free.
A five-way tie for fifth at four-under 67 featured Jason Caron, Tim O'Neal, Stewart Cink, South Africa's Retief Goosen and Australia's Mathew Goggin. Defending champion Joe Durant opened with a one-under 70.
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