The FMCG giant, which operates in 165 countries, has iconic brands in the UK including Cadbury, Oreo, belVita and Philadelphia.
Golin handles consumer PR for several of its brands, including Cadbury. Carat handles media planning and buying in he UK and Ireland.
Mondelez International trade communications manager Susan Nash said Hanover demonstrated "a great understanding of the food manufacturing and retail landscape".
Hanover head of trade Ella McIntosh said the agency was "thrilled" to work with Mondelez.
"They are one of the most important companies in the FMCG sector and having them on board, alongside Lucozade Ribena Suntory, gives us an incredible trade offering and positions us as one of the most important growing agencies in the sector," she said.