Formerly the executive creative director at Publicis New York, his new role will see him take overall control of the creative product in all of Vodafone's global markets.
His main responsibility will be to unite the account under a single creative idea that will be used and adapted throughout the network.
Craig Davis, the chief creative officer at JWT Worldwide, said: "Howard had the pick of the opportunities in London. He is one of those rare creative talents with the ability to produce brilliant work himself and a proven record of strong creative leadership.
"Vodafone operates in a fast and furious industry with incredible potential and Howard is a great piece of casting."
Willmott added: "Vodafone is at the forefront of technology and is a brand whose relevance to life makes it one of the most exciting and rewarding to work with."
John Rudaizky, the global business director on Vodafone, said: "Howard's leadership experience on both sides of the Atlantic will be invaluable as we work with Vodafone to take our creativity to a new level across a fast-changing media landscape. As we build our global brand, he will give us one idea and one tone of voice."
Willmott moved to Publicis New York in April 2003 from Saatchi & Saatchi in London. He switched agencies following an offer from Dave Droga, who had quit his post as the creative chief at Saatchis the previous year to become the Publicis worldwide creative director.
He joined Saatchis in 1994 and remained there until December 2000, when he joined Leo Burnett. However, he returned just four months later at the request of Droga.