According to Harvard, the brief is to build a profile for MediaCom in the UK in a period of "profound disruption" amid the growth of digital, working with UK CEO Josh Krichefski (pictured).
The account was previously held by The Media Foundry.
MediaCom, which is owned by WPP, is one of the biggest media agencies in the world, with a client list that includes Sky, DFS, Tesco and Cancer Research UK.
Claire Ferguson, MedicCom managing partner and head of agency communications, said: "This is an exciting but clearly challenging time in the media and we’re happy to have found the right agency to put us front-and-centre in this."
Harvard Group CEO Louie St Claire said: "It’s a fascinating time to be working alongside MediaCom, with the media sector, especially within advertising, experiencing seismic shifts – largely thanks to the impact and growing prominence of digital. MediaCom has a hugely important role to play in helping brands adapt, and we’re delighted to be working with a true leader in its field."