During his time at Craik Jones, Mitchell focused on business development, creating new revenue streams from existing clients. He was close to the First Direct account and worked on most of the agency's pitches.
Mitchell has been gradually reducing his hours at the agency since 2004, when he went part-time. He will continue to work for the agency on a consultancy basis, helping with business development and pitches.
His departure leaves David Watson, the Craik Jones chief executive, as the last remaining founder at the agency.
Watson said: "Robin and I have worked together for 20 years, first at Ogilvy & Mather, and latterly as partners at Craik Jones. Most marriages don't last that long. His broader marketing and business strategy skills are unrivalled, and I'm delighted that we'll still be able to tap into those, as well as giving Robin the freedom to work on interests outside the agency."
Mitchell said: "After 15 years, I want more time to pursue other interests. I'm really proud of what I've helped achieve here and I'm delighted that I'll be able to continue my involvement with the agency. I know I can rest easy in the knowledge that, in Fiona Scott and Jackie Stevenson, I've helped put a first-rate management team in place."
Mitchell began his agency career in 1985 at Ogilvy & Mather, after seven years client-side at Dixons and 3M. After three years at O&M, he was promoted to the board. He left the agency in 1991 to set up Craik Jones.