The network won the business after a pitch against Universal McCann, the UK incumbent on T-Mobile, and MindShare.
MediaCom previously held the Deutsche Telekom account in the domestic German market, but it picks up the £20 million UK account from Universal.
Universal won the T-Mobile business two years ago after it moved from Starcom Motive as part of a consolidation process.
However, the bad news for Universal was balanced by its capture of the £19 million More Than business after a pitch against the incumbent, Manning Gottlieb OMD, Carat and ZenithOptimedia's Zed division.
Universal has reached a three-year agreement with More Than and will work across media planning and buying for brand, direct response and online marketing. Creative stays with CCHM.
Lee Mooney, the head of marketing communications at More Than, said: "Universal McCann performed very strongly throughout the pitch process.
It brought some fresh ideas and new insights into our challenges together with a very strong team that will help us to meet the next step-change we are looking to achieve in our business."
Andy Jones, the joint managing director at Universal McCann, said: "Our task is very much to integrate More Than's response and brand activity. We believe we can really take that forward."
Commenting on the T-Mobile loss, Jones said: "We're very disappointed because it was purely driven by a series of tenders to procurement people."