The Southampton-based agency has opened a second office in the capital's West End and has restructured to provide greater focus on its core capabilities.
Gill, who was previously business partner at OgilvyOne Worldwide, has been brought in to broaden the agency's client offering. Gill looked after Unilever's global digital account at OgilvyOne.
Steve Sponder, managing director of Five By Five, said: "Five By Five is not only changing rapidly as a company but we also work in a very fast moving landscape where agencies must understand all the challenges facing brands in order to deliver profitable marketing solutions.
"We aim to deliver communications that stand out in the crowd, and to achieve this we are always evolving in a flexible and strategic way."
Other new appointments at the agency include Iain Trundle, who joins from Moore Wilson, as account director for web development projects, Nicola Cooper as senior account manager and Emma Holden as account manager.
Five by Five has recently won new business for Activision, Britvic, New Look, Gerber Juice and Sun International.