LA QUINTA, 23 Jan 2015 (AFP) - His one-stroke advantage came thanks to a blistering birdie run that included six straight from 12, with Italy's Francesco Molinari and fellow Americans Mark Wilson, Blake Adams, John Peterson and Scott Pinckney sharing second on 64.
It was a further stroke back to defending champion Patrick Reed, Matt Kuchar, Jason Kokrak and Steve Wheatcroft.
Putnam, 31, a three-time winner on the Web.com developmental tour, is seeking his first US PGA Tour win.
His assault came on the PGA West's Nicklaus Course, one of three in use for the first three rounds of the tournament in the southern California desert.
With birdies sure to abound all week, Putnam said he knew he couldn't afford to rest on the first-round lead.
"You have to mentally throw this one away because you're not going to play the Nicklaus (again)," Putnam said.
"The focus for me is on the Palmer tomorrow and then La Quinta on Saturday. There’s a lot of birdies going to be made in 54 holes by a lot of guys. So I have to make a lot of birdies."
Playing in his first US PGA Tour event since withdrawing from last season's BMW Championship, 42-time Tour winner Phil Mickelson carded a one-under 71 that left him tied for 89th.
Mickelson had three birdies in his first nine holes on the La Quinta course, but had three bogeys and a birdie coming in to lie eight shots off the lead.
"I got off to a pretty good start, and then I just stalled," Mickelson said, adding that he was looking forward to trying to improve on Friday.
"I can't wait to get started again because I feel like I played a little tight today, kind of steered it a little bit," said Mickelson, who lifted the trophy here in 2002 and 2004 but hasn't finished in the top 35 in his past two appearances.
Leading first-round scores on Thursday in the US PGA Tour's $5.7 million Humana Challenge (USA unless noted, par-72):
63 - Michael Putnam
64 - Francesco Molinari (ITA), Mark Wilson, Blake Adams, John Peterson, Scott Pinckney
65 - Patrick Reed, Matt Kuchar, Jason Kokrak, Steve Wheatcroft
66 - Pat Perez, Nick Taylor (CAN), Eric Axley, Erik Compton, Heath Slocum
67 - Brendan Steele, Harrison Frazar, Nick Watney, Bryce Molder, Brian Davis (ENG), Charlie Beljan, Brian Stuard, J.J. Henry, James Hahn, Harris English, Charles Howell, Bill Haas, Jason Bohn