Working for the Unilever-owned ice cream maker's brand development team, the agency is charged with getting people to 'love Ben & Jerry's and to love their ethics'.
Unity co-founder Nik Done said: 'Ben & Jerry's is an ideal Unity client - a big, brave exciting brand, but with an ethical bent.'
Unity's brief is to maintain the fun and quirky aspect of the brand while also increasing the company's ethical credentials among 18- to 34-year-old ABC1s. Ben & Jerry's will be fully Fairtrade-certified by the end of 2011.
Regular retained work will include ongoing efforts to promote the brand in the UK and Europe, reputation management and advising on trends.
In addition, the agency will provide European planning to support existing initiatives such as 'free cone day', covering PR, brand experiential and digital marketing.
The agency will also help plan for the launch of new ranges across Europe.
Ben and Jerry's currently also retains Mischief for UK PR.