The 46-year-old American posted six birdies with two bogeys, capping his four-under 68 with a 16-foot birdie putt at 18.
The 46-year-old American, seeking his first victory since the 2013 Open Championship, posted six birdies with two bogeys, capping his four-under 68 with a 16-foot birdie putt at 18.
He was among half a dozen players tied for the lead in the elite event, being played in Mexico City for the first time after the loss of a title sponsor saw it moved from Trump National Doral in Miami.
"This is an incredible challenge for all of us," Mickelson said of the high altitude course. "All the guys out here are so competent, so great at distance control that you are always seeing them pin-high, and it's a real challenge to do that here because of how far the ball goes.
"That's been the most fun challenge," added Mickelson, who was tied with compatriots Jimmy Walker and Ryan Moore, Spain's Jon Rahm and England's Ross Fisher and Lee Westwood.
Westwood was six-under through 16 holes, but finished with back-to-back bogeys to fall into the group.
They were one in front of another big bunch on three-under par 68 that included Northern Ireland's Rory McIlroy.
McIlroy, sidelined for seven weeks by a rib stress fracture, said he felt no ill effects during a round that included two birdies, one bogey and a 28-foot eagle putt at the par-five sixth -- his 14th hole of the day.
"The rib is fine," McIlroy said, although he added that he was coping with the effects of apparent food poisoning that had kept him awake much of the night.
"I don’t really want to tell you what I'm feeling right now," McIlroy said. "I'm definitely a bit weak. I haven't eaten anything since breakfast, so I'm a little weak."
Nevertheless, McIlroy was in positon to chase for a victory that could see him return to number one on the world -- should current number one Dustin Johnson finish in a tie for fourth or worse.
McIlroy and Johnson are grouped together, along with Japan's Hideki Matsuyama, for the first and second rounds.
Johnson, who moved to number one with his victory at Riviera Country Club two weeks ago, endured a frustrating day en route to a one-under par 70. He had six birdies, but missed seven putts within 10 feet and took a double bogey six at the par-four first hole -- his 10th -- when he hit his tee shot out of bounds.
"Obviously I didn't score very well at all," Johnson said. "But I played really well. I thought I hit the ball well, hit a lot of great iron shots, had a lot of good looks. I missed so many putts -- six of those seven were inside five feet."