The company also announced the promotion of MediaCom U.S.'s co-CEO, Dene Callas, to sole CEO of the U.S. media services agency.
Neither Mr. Mandel nor Ms. Callas were immediately available for comment.
Both moves come as Grey Global Group shops itself to buyers. Britain's WPP Group and France's Havas have expressed interest in acquiring the marketing communications company.
MediaCom Worldwide CEO Alexander Schmidt-Vogel, speaking by phone from Germany, where he is vacationing, said: "Dene will become the sole CEO of the U.S. and Jon will still be involved in giving advice, but his time will be spent more on focusing on international."
Mr. Schmidt-Vogel said Mr. Mandel was made chairman a few days ago, though in practice his global buying role had existed for a while. He declined to comment on the length of Mr. Mandel's new contract, but said all parties were happy with the terms.
News of the changes came in a company statement issued to the press this afternoon. Mr. Schmidt-Vogel said to expect more management changes around the world as part of a new company positioning that emphasizes the strength of MediaCom's people.
"There are drivers and there are passengers and we only want drivers," he said. "Loyalty is one thing, just loyalty doesn't work." Mr. Schmidt-Vogel said the company was committed to looking at the weaker countries and improving the people as part of the agencies' commitment to what it calls "People First, Better Results."