This includes E.ON's online media planning and buying account, formerly handled by Zed Media.
MediaCom pitched against Initiative, the incumbent, on E.ON's offline media planning and buying account, along with PHD and Starcom.
MediaCom already handles E.ON's media business in Germany.
Last month, E.ON hired MVi, the digital division of MediaVest Manchester, to handle its search marketing account.
Jeremy Davies, formerly the Abbey brand and communications director, was appointed the top marketer at E.ON in June. He succeeded Gary Pepler, who held the role on an interim basis since Helen Merrick left earlier this year.
E.ON, a German company, dropped the Powergen brand last year in favour of its corporate name.