The five-time major winner, who practiced at Augusta National for two days this week in advance of next week's Masters, was two shots behind early leaders Keegan Bradley and Matt Kuchar; both finished at 6-under 66. J.B. Holmes was also at 6 under after 17 holes.
Mickelson and Bradley were part of a threesome with Webb Simpson, who also finished at 4 under. All three players navigated the 7,441-yard Golf Club of Houston layout without a bogey.
Stewart Cink finished his morning round with a 5-under 67.
Quiet Please: Shell Houston Open, Prelim To The Masters
With no Tiger on the trail to the Masters, who will be impacted most, golf fans or Tiger's competitors? ESPN senior golf analyst Michael Collins analyzes this topic and gives his views of the Shell Houston Open and who will win the Masters.