The Dentsu Aegis Network agency beat Spark Foundry and Wavemaker to the post, which is estimated to be worth £50m. Mondelez called an international review of the account in March.
Carat previously also worked on the North America business but this moved to Spark Foundry earlier this year. The agency has been working on the UK account since 2015, after the last global media review by Mondelez.
Mondelez owns confectionery brands such as Cadbury, Oreo, Toblerone and Bel Vita.
A spokeswoman for Mondelez said: "Our current assignments are coming to an end. So, we just took a new look at our media investment to address some key industry changes such as programmatic, transparency and eCommerce and better equip our company for the future."