He said in a statement released to golf.com that he had improved a great deal after working with one of the sport's greatest teachers but that he was looking for a fresh approach.
"I’ve learned a great deal from him in our eight years together. It's just that at the moment I need to hear new ideas from a different perspective,” said Mickelson, who last recorded a victory at the 2013 British Open.
Mickelson linked up with Harmon in 2007 and notched up 12 PGA victories including the 2010 Masters and the 2013 Open.
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The change comes after two consecutive winless seasons, the longest drought of a professional career that includes 42 PGA Tour wins.
Mickelson said Harmon deserved a place among the greats of the game.
“Butch is one of the great teachers in the history of the game, and I believe he deserves to be in the World Golf Hall of Fame,” he said.
The 72-year-old Harmon, who has coached some of golf’s biggest names including Tiger Woods and currently has the likes of Dustin Johnson and Rickie Fowler on his books, said there were no hard feelings about the split.
“Helping him win the Open Championship in 2013 was one of the pinnacles of my career,” Harmon said in the statement.
“I see nothing wrong with him seeking advice from another source. We’re great friends and always will be.”
(Reporting by Ben Everill in Los Angeles; Editing by Peter Rutherford)
