As the director of digital, Sycamore will be based at Arc and will oversee the digital development of both agencies as well as sitting on their joint European leadership council.
Reporting to Andrew Edwards, the Arc UK chairman, Sycamore's job will be to ensure both agencies have the ability to provide digital solutions on all of their accounts.
He will also have specific responsibility for controlling the digital output on major European pieces of business such as Kraft, Procter & Gamble and iShares.
Edwards said: "The appointment gives us a much more strategic and creative power in digital. And because we try to run digital through everything we do, instead of having specific digital teams, he will ensure that we are following best practice as well as adding focus to our output."