The announcement comes after rumours of an ongoing dispute between Ryan and 20:20’s private equity backers Doughty Hanson.
Ryan’s role will temporarily be split between 20:20’s existing management teams and a search for his successor is expected to begin shortly.
A statement from the company said: "20:20 Mobile announces today that Mark Ryan is stepping down as Chief Executive with immediate effect.
“Mark has been with 20:20 for the past 10 years and has been an integral part of its successful development and growth.
“20:20 Mobile is now entering a new phase of growth as the Group expands internationally and further develops its value-added services for customers."