Day has become chief operating officer and will work with Amanda Pierce, Burson-Marsteller UK’s CEO, on the agency’s strategy and planning as well as deputising for her when needed.
He will also contribute to driving growth across the whole agency in addition to retaining his role as head of public affairs.
Day joined Burson-Marsteller from Portcullis, where he was managing director of its public affairs practice.
He began his career working for former Conservative shadow ministry David Heathcoat-Amery, and has also worked for FleishmanHillard.
Day said: "I have very much enjoyed the past two years as head of public affairs and look forward to working with Amanda and the team in my new role to build on the agency’s outstanding global reputation and further enhance our standing in London."