Tunnicliffe, the managing director of Western Media before its UK merger with Initiative, will take on the post of global client and network development director.
Tunnicliffe moved to the US last year, but has considerable experience in managing international accounts, including Unilever across 38 markets.
Initiative wants to use Tunnicliffe to target global new-business prospects.
Reporting to Initiative's chief executive, Alec Gerster, Tunnicliffe will also play a role in client servicing of global business.
Gerster said: "I have seen Mike's drive to deliver the best of the Initiative network against new-business opportunities across the globe. His knowledge of our network and the media business makes him the ideal choice."
Initiative faces the challenge of replacing losses including Unilever in Europe.