An announcement from M&S said that McWalter "has decided to leave to pursue other interests outside the company".
It came as several other changes were announced to the board. M&S said that it has now split the CEO and chairman's role, with Roger Holmes taking up the CEO role on September 1 and Vandevelde to go part-time as chairman from January 1 2003.
The shake-up will also see marketing lose its place on the M&S board.
McWalter joined M&S in 1999 from Woolworths, where he was marketing director. During his time at M&S, he oversaw the appointment of Rainey Kelly Campbell Roalfe/Y&R to handle television advertising, and launched the controversial "I'm normal" campaign, featuring a naked size-16 model.
Matthew Chambers, head of advertising for Marks & Spencer and the man directly responsible for the controversial campaign, lost his job last year.
While the campaign inspired religious groups to rip the posters off hoardings, it failed to achieve a turnaround in the group's fortunes and clothing sales continued to fall. However, a new strategy did later reverse those declines.
Vandevelde said: "I want to thank Alan for the significant contribution he has made to the recovery, the restructuring and the lead he has taken in the redevelopment of the Marks & Spencer brand."
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the .