Snedeker took advantage of the ideal conditions to fire a sparkling seven-under-par 64 at the TPC Scottsdale, finishing four shots behind the pacesetting Mickelson who lipped out with a birdie attempt for a 59 at the last.
"Obviously I needed to go a little lower," a smiling Snedeker said after his partners, Padraig Harrington (64) and 2010 champion Hunter Mahan (67), had also gone low.
"It was fun out there. We had a great group of guys, me and Padraig and Hunter had a great time out there today. We kind of fed off each other, got off to a hot start and played pretty well."
World number seven Snedeker and company had a perfect view of Mickelson's scintillating 60 on the Stadium Course after teeing off from the par-four 10th just one group behind.
"It gave us a target to go after," Snedeker said of watching Mickelson's flawless, 11-birdie display. "We're all fans of golf and we're all fans of Phil, so we wanted to see him make that putt on the last hole.
"We were listening for the roar on (number) eight green and didn't hear it go in, which was a shame. I heard he put a good roll on it."
Mickelson lipped out from 25 feet at the par-four ninth, his final hole of the day, and had to settle for an early four-shot lead before play was suspended in darkness with 33 players still out on the course.
Triple Major champion Harrington of Ireland, making his first appearance in the Phoenix Open, was happy with his opening seven-birdie 64.
"It's a little bit behind Phil but still a nice score in itself," the Dubliner said. "I pretty much got the most out of the round for the first 15 holes, then had three chances the last three holes and didn't hole the putts.
"But overall I've got a good feeling about it. I know there's going to be a couple of errors here and there. Hopefully, I can keep making enough birdies to counteract that.