MediaCom has yet to confirm which key clients Rotherham will work across when he joins the WPP agency in October.
He joins from OMD, where he previously headed the Omnicom agency's global Vodafone business.
Rotherham will be based in the London office and will report to Christian Schmalzl, MediaCom’s chief operations and investment director.
MediaCom said Rotherham will, "take management responsibilities for some of [the agency's] biggest global accounts, ensuring development of strategies that will continue to deliver innovation, digital integration and, most importantly, business results," and guide local teams in deploying global strategies.
Stephen Allan, MediaCom's worldwide chairman and chief executive, said: "We are committed to attracting the best people to our company and Ian is a perfect reflection of our goal to recruit world-class talent in the industry to service our global clients.
"Undoubtedly he is one of the best global account directors in our business."
For the past three and-a-half years at OMD, Rotherham was EMEA management team member.
Before joining OMD, he was head of international at MPG where he relaunched the international division of the agency.
Prior to this, he was vice-president at MTV Networks Europe, where he was responsible for the European advertising strategy of MTV.
Schmalzl said: "Ian has proven he is a world-class leader in rolling out efficient and effective global processes across multiple markets. His experience will no doubt ensure we continue to drive innovation and creativity for our key clients."
Rotherham said: "Many of MediaCom’s key global clients have been with them for a very long time – over 30 years in many instances – and I think this is testament to the work and commitment they deliver to their clients."
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