His departure comes one month after it emerged that the agency's executive creative director, Jeremy Craigen, was moving into a role heading up the network's global Volkswagen account.
It is understood that Fox could remain within the Omnicom group. It is not known if he will be replaced at DDB.
Fox joined DDB's UK agency in 2006 as the managing director. He was previously the senior vice-president of DDB Europe, where he was instrumental in running the VW account.
He was later promoted to chief operating officer and then the new role of chief client officer across the group.
Last year, DDB UK, which is led by the chief executive, Stephen Woodford, was hit by the loss of Virgin Media after just 18 months working on the account. It is looking for a replacement for Craigen, who is expected to move to his new role later this year.