Lander, group director of comms at the international public transport group, has stepped back from day-to-day responsibilities with immediate effect.
However, she will remain with the firm to oversee a smooth transition during a period of gardening leave until 30 April 2011.
The departure is a result of National Express' decision to reshape its comms department around the firm's business units, rather than keep a group-level team, although exact details remain under discussion.
John Devaney, chairman of National Express, said the firm was 'sorry to lose' Lander and that she had made 'significant progress in rehabilitating the brand with key opinion formers'.
Lander joined National Express in June 2008 from Dairy Crest, where she was director of comms for two years.
She has also held senior comms roles at Woolworths and Tesco, and was named PRWeek's PR Professional of the Year in 2003.