That left Mickelson in a share of 32nd place, four strokes back of Lahiri and Americans Jason Dufner, Jeff Overton and Jerry Kelly, who were level at eight-under par 64.
Dufner, looking to rediscover the form that won him the PGA Championship in 2013, capped a flawless round with a birdie at his final hole, the par-four ninth, to grab a share of the lead.
Americans Colt Knost and Jamie Lovemark were a further shot back.
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Mickelson eagled the par-four eighth after his 110-yard approach hit the flag and spun right back into the cup. The eagle, along with a trio of birdies on the front nine, put Mickelson near the top of the leaderboard.
He went on to pick up another stroke two holes later but fell back after bogeys at the 14th and 17th holes.
"It was a fun start to the year. I've had a lot of time off, it was fun to get back into the swing of it," said Mickelson. "I had a good solid round."
Lahiri, who last October became the first golfer from India to play at the Presidents Cup, carded eight birdies in a flawless round at La Quinta Country Club, one of three co-hosting venues.
Defending champion Bill Haas made a rousing start by holing out his second shot for eagle at the par-four first before going on to card a six-under 66 to sit in an 11-way tie for seventh.
(Reporting by Frank Pingue in Toronto; Editing by Larry Fine)
