Yamaguchi, who spent 28 years at Dentsu, managed the agency's subsidiary agencies in Europe and the Americas as deputy director of the international management division.
Simon Sherwood, chief operating officer of BBH Group, said: "'Kat' is a rare species. Although he has spent his entire career working in Japan, he has a highly developed understanding of the global marketplace. He is completely at home both inside and outside of Japan. To have found him is in itself surprising, but to have persuaded him to join BBH as CEO is something we are very pleased about."
At Dentsu, Yamaguchi developed alliances with Publicis, Dentsu's major overseas partner, handled mergers and acquisitions and client support.
During his career he has worked with clients including McDonald’s, United Airlines, BMW, Coca Cola and GlaxoSmithkline.
