Manwaring will take the role of communications director, which was vacated by Tim Allnutt. Allnutt resigned to join Naked Inside, a joint venture between Clemmow Hornby Inge and Naked Communications.
Manwaring's main task is to develop Initiative's communications planning offering and enhance its strategic output.
Since leaving Initiative to launch his own consultancy, the Manwaring Partnership, Manwaring has provided communications advice to companies including BSkyB, Unilever, IBM and Yahoo!
Jerry Hill, the chief executive of Initiative, said: "It's fantastic that we have been able to entice Tony into this role. He is arguably one of the most respected strategic and creative brains in the business and will bring increased experience at a time when the agency is going through such rapid expansion."
Manwaring is expected to move fully into the role in four to six weeks' time. He said: "I've had a blast, having the opportunity to go and do my own things. I set up my own business, raised money in the City and now want to use my experience in a proper job.
"The role is extraordinarily powerful and consists of putting together our services in a way that is relevant to clients. It's not a rebuilding job but re-engineering of what is already there."
Manwaring was previously at Initiative for ten years and has also held roles at Publicis and Lowe Howard-Spink.