Lang, who has been in the role since 2001, is to return to consultancy from the end of February, although she is in talks with AOL UK chief executive Karen Thomson about continuing to work with the company on a number of unspecified projects.
Her departure comes as AOL significantly boosts its marketing budget to establish itself as the leading UK broadband provider.
Last year it spent more than £42m on advertising, according to Nielsen Media Research. It expects to exceed this figure in 2004 after stepping up promotion of its broadband services, with an increased focus on TV.
AOL is not expected to replace Lang directly. Since her resignation, it has been reviewing the structure of the marketing department.
It is also continuing its search for a brand director, with an appointment expected before Christmas.
Lang, who featured in Marketing's Power 100 Next Generation list, was responsible for hiring Grey London at the end of last year to handle AOL UK's advertising business, replacing long-term incumbent Mortimer Whittaker O'Sullivan.
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the Forum here.