Morris is leaving the agency after 26 years. A statement from Miller said: "We've concluded that it is in the best interests of the agency to make a change of leadership."
Miller will not be replaced as the vice-chairman, but Tom Lehr and Mike Perdigao, who are both executive vice-presidents and managing directors, will take on additional responsibilities.
Cliff Einstein will remain as the chairman and still runs the agency's regional Ford dealer business, while Steve Rabosky continues as the president and chief creative officer.