In search of his first Open victory, Mickelson fired a 2-under 69 and was three shots back of early clubhouse leader Zach Johnson.
Mickelson, who won the Scottish Open on Sunday, started strong, carding birdies on two of the first three holes before dropping a stroke at the par-4 eighth. He again rolled in consecutive birdies late in the round.
"I played well," Mickelson said. "I had a spurt there in the middle of the round where I had some technical difficulties and hit some poor shots but was able to make pars and then turn it around. I kind of keyed in on it after the par [at] 13. And from there on in, I hit a couple of good shots and made a couple of birdies coming in."
He did leave a few strokes on the course, however, after narrowly missing a birdie at the par-5 17th and following that with a three-putt bogey at the par-4 finishing hole after a cozy approach set up a makable birdie putt.
"You know, we've all been there where you make a bogey ... to try to get that birdie back," Mickelson said. "And, sometimes in trying to get it back, you put it in a spot that you make another bogey."
Still, Mickelson will take this effort as opposed to last year's start, which ended with him missing the cut and finishing 11 over. Perhaps this year will instead mirror Mickelson's 2011 Open Championship, when Lefty tied for second in his best finish in a British Open.