Mickelson shot 4-over-75 and is three-over-par for the tournament at Pebble Beach, Calif. The six-time U.S. Open runner-up ended play tied for 48th.
Mickelson said he felt underwhelmed despite hitting a 370-yard drive on the ninth hole.
"I felt like I played a lot better than I'm scoring," Mickelson said. "And [I] certainly didn't finish this round very well."
Mickelson birdied the 12th hole and sat at two-under. He recorded bogeys on holes No. 11, 14 and 17 before hitting his tee shot into Carmel Bay on the 18th.
"I'll come out tomorrow and play as well as I can," Mickelson said. "But today was the day I needed to go low. I played well enough to do it. .... And then I finished poorly."
Mickelson celebrates his 49th birthday Sunday.