The pair have worked together since 2007 and Mickelson won the 2010 Masters and 2013 Open Championship titles during their spell together.
He also won another 10 PGA titles but he has failed to repeat his best form in recent months.
After winning the Open in 2013 at Muirfield, he has finished in the top 10 on just five occasions and the American has confirmed he will look elsewhere for advice on his game.
He said in a statement: “Butch Harmon 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.
“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.”