The network has confirmed that he will step down in June as chairman of its Europe, Middle East and Africa operations.
Giueseppe Usuelli, the chairman and chief executive of McCann Worldgroup Italy, will replace him.
John Dooner, the network's chief executive, praised Howell for restoring confidence and growth in the region after a difficult period. "We wish him all good things," he said. Howell joined McCann from Chime in 2003.