Mullen arrives at Reach from Ladbrokes Coral where he also served as chief executive in a career which includes product management and digital strategy roles at News International as well as 11 years in the advertising industry.
Fox said: "There is never an ideal time to leave an organisation, but if there were it would be now. The integration of the Express and Star has been completed, digital growth is accelerating and our trading and cash position are strong. I am proud of what has been achieved and will provide Jim with whatever support is required to ensure a smooth handover."
Jim Mullen said: "I am delighted to be joining Reach at such an exciting time and look forward to building upon its digital transformation".
The handover of power coincides with negotiations by Reach to acquire the assets of JPI Media, formerly Johnston Press.
Fox was first appointed to run the UK's largest regional newspaper publisher in August 2012, having previously run HMV and Waterstones.