Davies was Microsoft UK head of PR for nearly three years before joining mobile giant Three in 2006, spending nearly nine years with the company, latterly as corporate affairs director.
Earlier in his career he had worked for Cohn & Wolfe and Leo Burnett. Since leaving Three in 2015 he has worked as a consultant.
Davies is tasked with building the firm's clientele in the tech, media and telecomms clients.
Hydra was set up by managing partner Stephen Benzikie, a former MD of Bell Pottinger's corporate & financial practice, at the start of the year, and was named one of PRWeek's new agencies to watch for 2017. It works across corporate, financial, investor relations and other areas.
Having launched with clients including Choice Hotels and investment firm London Asia Capital, it now has 15 staff and recent wins include private equity trade body Invest Europe and energy firm Encor Power.
Davies said: "In a more agile world, clients want access to experienced people without significant overhead costs. I believe Stephen is building a business that is both flexible and sustainable through Hydra's ability to deliver incisive advice from very experienced communicators to its growing mix of clients."