Wall, who quit Fallon last May to take a sabbatical, will report to Joao Wengorovius, the chairman and chief executive of the BBDO group in Portugal, and Pedro Bidarra, the executive director and creative chairman, who previously ran the agency together.
Wall has been brought in to take control of the business while Wengorovius and Bidarra focus on running and expanding the group offering.
William Eccleshare, the chairman and chief executive of BBDO EMEA, said: "We take pride in getting an unfair share of people who have an unfair share of talent. It is a great creative agency, and he has a lot of experience in running those."
Wall, who moved to Lisbon in 2006 and speaks Portuguese, added: "I decided ages ago to move here, but had no definite plans about work. However, this industry is so addictive, you need to be involved, and this offer came along at just the right time.
"The agency leads the market in creativity and has some great accounts. And BBDO is a great club to be in."
Wall launched Fallon London as a founding partner alongside Robert Senior, Laurence Green, Andy McLeod and Richard Flintham in 1998. In 2004, he was promoted to president of international, a position he took in addition to his London responsibilities.