Miller has run the agency for the past six years but has left without a job to go to.
The news follows Wieden & Kennedy's loss of some of its flagship Nike EMEA business. Last month, Nitrosoul won a pitch against Wieden & Kennedy Amsterdam for a pan-European branding project. Paul Shearer, a Nitrosoul partner, was a creative director on Nike at Wieden & Kennedy for two years.
The agency has also struggled with the Vodafone account, losing lead creative status to Bartle Bogle Hegarty last year.
Miller, who reported to Dave Luhr, Wieden & Kennedy's chief operating officer, left the agency last week. Wieden & Kennedy is now recruiting for a replacement.
Nobody at the agency was available to comment.
Miller has worked at Wieden & Kennedy for seven years. He joined the Amsterdam office in 1999, replacing John Russell. Before that, he was the managing director at the Wieden & Kennedy-McCann Erickson alliance in Tokyo. He has also worked as an account director at Leo Burnett in Tokyo.