Grainger will work across all clients including Tesco and Amazon. He will report to the managing director, Anna Watkins, and help to develop the agency’s "Barefoot Running" positioning.
Grainger spent 14 years at ZenithOptimedia, rising from the position of senior research executive.
Watkins said: "David’s experience, client focus and first-class strategic capabilities will add a tremendous amount."
Regan stepped down from Initiative in November last year. Since then, the chief executive, Alex Altman, has also left the agency as part of a pan-regional restructure.