Harrington, a three-time major champion who has not won in the United States since capturing the PGA in 2008, posted the lowest 18-hole total of his career.
Will Claxton, who posted a top-20 finish last week in Puerto Rico, shot a 64 to stand alone in second place.
Harrington birdied three of the first four holes and finished with three straight birdies. His round was highlighted by a 75-foot birdie putt on the par-3 17th.
"Must be the longest putt I've holed on tour," Harrington said. "I would like to tell it you was funneling right up the green and couldn't go anywhere else, but certainly the last 15 feet it was right in the middle of the hole. That's a bonus, obviously."
Former U.S. Open champion Lucas Glover, making his season debut after injuring a knee prior to the start of the campaign, shot a 70.
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