The Cardiff-based agency beat three other agencies to win an annual contract with the Welsh bottled mineral water producer.
The account was previously held by JWT Cheetham Bell for a period of just over 12 months.
Cake Communications starts work on the account this month, managing national PR and social media as well as supporting existing sport sponsorship deals with Run Wales, Cardiff Blues and Cardiff City.
The agency’s campaigns will focus on the brand’s Welsh heritage and highlight the importance of water in a healthy lifestyle. Account manager Martin Sparey said the main aim would be to put Brecon Carreg at the front of people’s minds in Wales. He added: "We want to position Brecon as the number one go to brand for Welsh consumers."
The account will be headed up by agency manager Ruth Walters, who previously worked at Hotwire and Weber Shandwick on clients including Samsung, Microsoft and Johnson & Johnson.
Sara Robinson, founder and managing director of Cake Communications, said: "We’ve matched a team to this client that not only truly believes in the brand and its ethos but is made up of the most active members of the team, never seen without a bottle of water."