The news was first reported Wednesday night by Golf.com.
Mickelson, who began working with Harmon shortly before the 2007 Players, won 12 PGA Tour events and two international tournaments during that span, including the 2010 Masters and 2013 Open Championship.
That victory at Muirfield represents the 45-year-old's most recent title. Since then, Lefty has posted just five top-10 finishes while slipping to No. 25 in the world. To play in last month’s Presidents Cup, his 21st consecutive team competition, he required a captain’s pick.
Golf Channel insider Tim Rosaforte reported that Mickelson flew to Las Vegas last week to deliver the news to Harmon and that the split was "amicable."
“I respect him as a person and a teacher and as a friend,” Mickelson told Rosaforte, “and just wanted to talk to him in person about it. It’s not something you do over the phone. He’s been good to me long before we ever started working together. That stuff is never easy, but it’s what he deserves.”
Mickelson told Rosaforte that he has a new coach in mind but wasn’t ready to reveal his next step publicly, out of respect to Harmon.
“I completely agreed that sometimes you need to hear things a different way, get a different perspective on things,” Harmon told Rosaforte. “He’s been frustrated the last two years. I thought it was a good idea that he would do this. He needs to hear things differently that maybe get him rejuvenated and get him back to what we all know he can be.”
Prior to working with Harmon, Mickelson spent most of his pro career with instructor Rick Smith.