The move comes as part of a change in direction after the departure of Pippa Stockdale, who was managing director of the combined London operation.
"Jon has an outstanding track record in the auction business. His career as both a dealer and auctioneer spans over 30 years," said CEO Matthew Girling.
Mr Baddeley, who will continue to oversee the collectors' sales, set out his aims in his new role, saying: "My ambition is not only to substantially grow the core business of jewellery, silver, paintings, furniture and works of art, but also to make Knightsbridge the centre of excellence for our specialist sales of portrait miniatures, arms and armour, coins and medals, science and technology, toys and dolls, and entertainment memorabilia."
Having begun his career at Sotheby's Belgravia in the early 1970s, Mr Baddeley became Head of the Collectors' department and auctioneer by the age of 25. He later left to become an independent dealer in decorative arts, but returned to Sotheby's in the early 1980s, remaining there for the next 20 years, finally becoming Group Head and Board Director of Sotheby's, London.
In 2003, he joined Bonhams as Global Director of Collectors' sales.
