Mickelson missed a short par at the closing hole for 73, his third over-par round for the tournament. After opening with 71-76-77, he finished with a 9-over-par 297 total.
Lefty posted his worst Masters finish since 2007, the only other year as a professional in which he recorded three over-par rounds at Augusta National.
“I just had an off year,” he said. “I don’t know what to tell you. I played poorly.
“This is disappointing for me in that this is my favorite place in the world to play. This is my favorite place to be. This is my favorite tournament and the one I look forward to the day after it ends. And to perform like this is disappointing.”
The three-time green jacket winner, who won the Phoenix Open in February, was one of the pre-tournament favorites. Despite hitting 71 percent of fairways over the tournament, Mickelson struggled with his iron play. He hit just 30 of 54 greens in regulation over the final three rounds.
“I’ll have something to work on the next two weeks before I start playing again,” he said.
Because of the change to the PGA Tour season this year, the Masters Tournament was moved back a week. Mickelson, who usually plays the week before major tournaments, played in the Houston Open two weeks ago but skipped the Texas Open. He said he has to figure out whether to resume playing the week before majors.
“I’ve got to look at that because the things I did this week to get ready I just wasn’t as mentally sharp as I need to be,” he said, “and I’ve got to find another way to get ready for big events if I’m not able to compete the week before.”