The appointment will enable TUI Travel, the company created by the merger of First Choice and Thomson last year, to launch its first significant campaign, having previously consolidated its creative account into Beattie McGuinness Bungay (BMB).
The firm shortlisted MediaCom and Carat for the final round of pitching, before selecting the former to work alongside BMB.
MPG, which handled Thomson's £7m media account, and Walker Media, which handled the £5m First Choice work, were knocked out along with Universal McCann in an earlier round.
The review was handled by Tim Williamson, director of product and marketing at TUI Travel, who previously held a similar position at First Choice.
Andrew Laurillard, head of brand strategy at TUI, said the company was now working on a campaign to break in the latter half of this year or early next year.
He added: 'The agency will be tasked with consolidating Thomson and First Choice as the preferred number one and two in the UK market.'
MediaCom's appointment follows a year of significant developments within TUI, whose merger came just weeks after Europe's second- and third-biggest travel operators, Thomas Cook and MyTravel, also joined forces.
The combined businesses of TUI Travel encompass more than 170 brands.