Clark, a former chief executive of Mindshare UK, will take up the Maxus role next month and will relocate from London to New York as part of the change.
Part of his task, once he has completed work on Group M's EMEA budgets for 2009, will be to identify growth opportunities for Maxus in new markets including the UK.
However, he denied that any plans for a UK Maxus launch were imminent.
He said: "I have had no chance to think yet about the UK or other markets where we don't already have a Maxus presence."
Maxus, which first launched in Asia in May 2004, is WPP's fourth media network alongside Mindshare, MediaCom and Mediaedge:cia.
It has built up regional strength across the Asia-Pacific region, Australia, India, Nordic markets and the US. Clients include GlaxoSmithKline, Shell and Miramax.
Clark, a US citizen who has also spent time working for WPP in Asia, was the chief executive of Mindshare UK until his move to the Group M role in late 2005.