Johnston began working for the 16-year-old from Hawaii when she made her pro debut last October in the Samsung World Championship, where Wie was disqualified over an illegal drop in the third round.
A public relations firm in New York released a statement from the family that said Johnson's departure 'comes as part of Michelle's maturation as a golfer, part of which is learning from many different bright golf minds.'
Johnston did not immediately return a message left on his cell phone. The Wie family was on their way home from England and could not be reached for comment.
Golf World magazine reported on its Web site that Wie's agent told Johnston at the airport Monday in Manchester, England that he had been fired.
'I was shocked and surprised. I thought we had a successful year,' Johnston told golfdigest.com. 'And I was extremely disappointed that no one named 'Wie' gave me the news.'
Johnston previously caddied 12 years for Juli Inkster and was on the bag when she won four of her majors and qualifed for the World Golf Hall of Fame.
Wie also used prominent PGA Tour caddies such as Fanny Sunesson, Mike 'Fluff' Cowan and Jimmie Johnson while she was an amateur. She is not scheduled to play again until Sept. 7-10 at the Omega European Masters in Switzerland, a men's event on the European Tour.
She also is scheduled to play the 84 Lumber Classic the following week on the PGA Tour, which will be her sixth attempt to become the first woman since Babe Zaharias in 1945 to make the cut on the PGA Tour.