Twohill will report to Ruth Allchurch, managing director of Cirkle, and sit on the company's management team. Twohill will lead the agency's digital team which includes Carli Goodfellow, director of digital influence.
At Havas, where Twohill has worked since 2011, she has been responsible for setting up a new social media division and developing the agency's digital metric model. She has experience working with clients including Google, Evian, Lemsip and Durex.
Allchurch said: "This was a particularly challenging role to fill as we were not only looking for a senior digital expert, but someone who had experience of PR from an integrated perspective.
"We consider ourselves exceptionally lucky to have found Claire; she will be a tremendous asset. Her dynamism, gravitas, strategic mind and industry knowledge will bring an added layer of expertise to the agency to help continue to grow our already healthy digital income stream."
Twohill is the latest senior appointment at Cirkle. In June the agency announced it had hired Kate Blakeley, Diageo's former head of reputation management for Western Europe, to the newly created role of deputy MD.