Mr Adamson, who joined Burger King in July 1993, is joining the Flagstar group, owner of the Denny's and Hardee's chains, as chief executive. When he became chief executive of Burger King Mr Adamson replaced Barry Gibbons, who was chief executive when Grand Met acquired the chain as part of the Pillsbury deal in 1989.
Commenting on Mr Adamson's departure, Grand Met said there was nothing sinister and that he had simply been offered what he considered to be a better job.
David Nash, chairman and chief executive of Grand Metropolitan's food sector, will take over while the group seeks a replacement.
Mr Nash said that Mr Adamson had helped to turn Burger King around. Sales are up around 6 per cent and profits have increased by 28 per cent. Burger King now has 7,500 restaurants and worldwide sales of £4.9bn.
In recent years Burger King has been conducting a back-to-basics campaign to concentrate on a menu of hamburgers, chips and cola. Its "value menu", which was pioneered by Mr Adamson, will remain, the company said.
Burger King caused a ripple of concern in December when it emerged that Grand Met had included the sale of Burger King restaurants to franchisees in its operating profits.
At £64m, this accounted for more than a quarter of Burger King's profits.