Middlemas will head an initial team of three, which includes former marketing and new business director Jennie Soffe and the recently appointed Ashley Evans as business development executive. The agency hopes to add a fourth person later in the year.
Andy Jones, chief executive of UM, said Middlemas' industry knowledge, experience and contacts, coupled with his "boundless energy", made him "the perfect candidate".
As well as collaborating with UM's agencies across the UK, he will work closely with the European business development team, led by Theresa Houlihan, and with UM's New York-based global team.
Middlemas will also continue his responsibilities for the agency's radio team and media lab presentation facility.
Separately, the agency has undergone a global rebrand, with all Universal McCann signage replaced with "UM". UM has also informed media owners that its holding company has changed from McCann Erickson to Media Brands.