Rees, 44, has been appointed to oversee a growth plan across the agency's trolley poster and field marketing business.
Jessica Hatfield, the founder of the company, is stepping down as its chief executive to take on a business-development role, focusing on format innovation.
Rees left MindShare two years ago, after joining the agency in 1998 from Carat. Since then, he has done some consultancy work but has not had a permanent job.
Established ten years ago, The Media Vehicle offers oportunities across retailers including Tesco, Sainsbury's, and Asda. Clear Channel owns a 29 per cent stake and Rees will also receive equity.
Roger Parry, the chief executive of Clear Channel International, said: "I have known Simon for many years and am delighted he has taken over."
Rees said: "I have joined the company to develop growth opportunities both in the UK and internationally."