Map, which is led from the US by global chief executive Brendan Moorcroft, combines search, social, mobile and display capabilities, and will serve both IPG media agencies, Universal McCann (UM) and Initiative.
Carter moves into the newly-created role of managing director, Map UK, from his position as managing partner, digital at UM London.
He will retain his lead role on UM London's Microsoft business.
Reporting to Arun Kumar, president of MAP G14, Carter will expand the business's capability in social and mobile and build on the existing search marketing operation Reprise and the programmatic display operation Cadreon.
Kumar said: "Our wish to create a unique, differentiated product through the Mediabrands Audience Platform depends on strong leadership to carry it through.
"That is why I'm extremely glad that Jason is taking up this role."
Follow Daniel Farey-Jones on Twitter @danfareyjones