Longworth was promoted to the post -- one of the most coveted roles in the industry -- in December 1999 following a 20-month search to replace Mark Sims, who resigned in April 1998.
Kellogg would not comment on speculation on when he left and whether he left of his own accord.
However, a spokeswoman for the company said he had "left recently to pursue other interests".
It is not thought that Kellogg has a replacement lined up, although traditionally the company has appointed internal candidates for its senior positions.
Before Longworth was promoted, UK marketing manager Joe Ennen had been touted as the most likely successor to Sims until he resigned in June 1999.
It had been expected to appoint Ennen to replace Sims until he resigned in June 1999.
Whoever takes over from Longworth will be charged with the task increasing sales of Kellogg's cereal brands, such as Corn Flakes, Nutrigrain, Bran Flakes and Special K, which have lost market share at the hands of increased competition from rivals Weetabix and Nestle.
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the .