Tobias Meyer, the chief auctioneer for Sotheby’s, has been the public face and a deal-making catalyst for one of the world’s most powerful auction houses.

As head of its contemporary art department, he has pursued and persuaded collectors to let him sell their masterpieces. He auctioned Edvard Munch’s pastel, “The Scream,” for nearly $120 million last year. And last week he sold Andy Warhol’s “Silver Car Crash (Double Disaster)” for $104.5 million.

But on Friday, the company announced that Mr. Meyer was leaving by mutual decision. He said he wants a career change that will put him on the opposite side of the auction podium he has dominated for 20 years. He said he intended to continue working with collectors, only this time as a private dealer.

Sotheby’s tried to play down the loss, calling to assuage important clients and minimize the impact, according to dealers in the field. But the news jolted the art world, where Mr. Meyer had the ear of many powerful buyers, including the publishing magnate S. I. Newhouse Jr., the fashion designer Tom Ford, the Taipei businessman Pierre Chen and the entertainment mogul David Geffen.