A week before the start of the year's first major, Mickelson recorded four birdies during a bogey-free round in ideal morning conditions at the Golf Club of Houston.
Mickelson, a perennial contender at Augusta National, pulled out of the last week's Texas Open during the third round with a pulled abdominal muscle.
It was merely a precautionary decision, because the three-times Masters champion Mickelson was at Augusta National early this week for a practice round on his way to Houston.
"I felt great today," Mickelson told Golf Channel. "I didn't feel any pain or discomfort and didn't even think about it.
"I'm surprised, because I was worried when it happened about the Masters but it healed a lot quicker (than expected)."
Mickelson was two strokes behind American leaders Matt Kuchar, Keegan Bradley and J.B. Holmes with half the field back in the clubhouse.
Kuchar, who was tied for the lead midway through the final round at the Texas Open before fading to finish equal fourth, bounced back quickly as he works towards Augusta.
"I was 18 for 18 for greens in regulation," said the six-times PGA Tour winner, who picked up three early birdies before adding three more in consecutive holes around the turn.
"I made a couple of nice putts mid-round to really get things going and missed a few coming in."
The Houston Open has attracted a quality field that includes 23 of the top 50 players in the world, and 46 competitors who have qualified for the Masters.
Among former Masters winners playing this week are Angel Cabrera, Charl Schwartzel, Trevor Immelman and Mike Weir, though 2013 champion Adam Scott is absent.