Mitchell, who was promoted to the role in 2009, is thought to be on gardening leave while he negotiates a departure settlement with the company.
Sources suggested that Mitchell's departure is linked to a programme of cost-cutting implemented by the Rapp group chief executive, Marco Scognamiglio.
Mitchell, who previously ran Rapp's media offering, had responsibility for the day-to-day running of the agency's London office, and worked on the integration of its creative and media services. He also launched Rapp's innovation unit, RaPPLabs, which is led by the group chief technology officer, Steve Furminger.
His exit follows more than 14 years' working for Rapp. Before joining the agency, then WWAV Rapp Collins, in 1997, Mitchell worked at Optimedia.
Mitchell was unavailable for comment.