The two agencies, part of WPP's GroupM media operation, won the business after a final shootout against Initiative Media.
Shell shortlisted the two agencies last month after Carat and Starcom were eliminated in the latter stage, while OMD declined to pitch as it has a global relationship with Exxon.
MediaCom and Maxus pitched as Team Media for Shell, which will be operated out of MediaCom UK in London.
According to Shell, WPP's Team Media for Shell successfully demonstrated a "great understanding of Shell's business, combined with a cohesive global media team".
WPP will take over Shell's WPP media from July 1 and will be responsible for all media buying and planning on behalf of Shell's global retail, global lubricants, including Pennzoil and Quaker State brands, corporate and human resources businesses.
Steve Allan, group CEO of MediaCom UK, added: "This is a tremendous win and accolade for Team Media for Shell. We are delighted to be working globally with one of the world's greatest brands."
Shell began the review of its account in January with a view to consolidating the business into one network. The UK account, worth around £6m, was already held by MediaCom.
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