Campbell takes over from Graham Jenks, who joined Abbott Mead Vickers BBDO as creative partner in January.
Gareth Leeding, creative director for sports, has been leading the agency’s creative on an interim basis since Jenks left at the end of last year.
Campbell is the agency’s first executive creative director in London. He left Guardian Labs in July last year after two years.
Since then, he has been working for Blood Global, which is part of You & Mr Jones, as a freelance creative director.
Campbell has also worked at WCRS and Agency Republic/Critical Mass. At We Are Social he will report to Jim Coleman, the agency's UK chief executive.
Coleman said: "[Alistair’s] focus is to guide us in creating even more inspiring work for our clients and to make sure that we remain a world-class agency.
"Alistair’s wealth of experience from agencies across the marcoms disciplines makes him a perfect fit."