The business, led by Carter and supported by a small team of consultants and partner organisations, will focus on "creating simple, compelling and impactful messages".
Carter left his role at Burson-Marsteller in June after joining the agency in 2010.
Message House will target CEOs, comms and brand directors that need research and expertise to help them construct effective campaigns.
Carter said: "The message is at the heart of any successful campaign and that will be the focus. When it comes to business, most are not short of messages, but this is about refining and crafting campaigns so they are impactful. I don’t think it’s something that there is enough time being spent on at the moment."
Carter was general secretary of the Labour Party from 2004 to 2005, helping lead political campaigns ahead of the 2005 general election in which Tony Blair secured a third term as Prime Minister.
He went on to set up the EMEA office of research outfit and Penn, Schoen & Berland before joining WPP stablemate Burson-Marsteller.
Carter will also continue to support both these agencies as a senior consultant.