As the worldwide creative director, Harrison will be charged with raising the standard of the creative product across the Wunderman network by introducing methods used at HTW in London.
In his new role, Harrison will spend about nine weeks a year in the network's key strategic offices in markets such as New York, Mumbai, Frankfurt and Buenos Aires.
In addition, he has been named the chairman of the London agency, which is merging with the WPP digital subsidiary Wunderman Interactive.
Jon Williams, the Wunderman Interactive creative director, and Nick Stewart, its head of planning, will become part of a newly created management board. They will join Harrison and the HTW joint managing directors Suzanne Partridge and John Hiney. Williams and Stewart will report to Harrison.
Harrison will continue to report to the Wunderman chairman and chief executive Daniel Morel. Morel said: "Since taking over at HTW, Steve's agency has produced the best work in the toughest creative market in the world. He has also dominated the world stage at Cannes. And as a network, we are determined to replicate this brilliance."
Harrison added: "It is the worldwide role that fascinates me. People have sometimes suggested that HTW would be consumed by the corporate nature of Wunderman. I have always maintained that HTW's creative culture is strong enough to permeate Wunderman. Global domination? Why not?"
HTW is among the most consistently awarded agencies in the world. At Cannes, Harrison has become the most-awarded creative director in direct marketing since the Lions Direct were launched in 2002.
HTW won the inaugural Lions Direct Grand Prix, and, in the four years since, it has won three gold, five silver and two bronze Lions for its work for clients including IBM, Star Alliance and Microsoft.
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the Forum.