Mr Mirfin has worked for Key for 22 years and will remain a major shareholder and continue to work for Key on a consultancy basis.
He is a well known media commentator on later life planning and equity release.
Key Group was sold to Partners Group in 2017 for £208m and has nearly 600 employees.
Mr Mirfin has been reducing his hours ahead of his change but will continue to sit on behalf of Key on the board of the Society of Mortgage Professionals (SMP).
Mr Mirfin said: “Having worked for Key for much of my career and helped it grow into one of the leading organisations in later life lending with almost 600 employees, now is the time to step back and concentrate on other opportunities.
“I will still be working with the senior management team at Key on a consultancy basis but I will look to share my experience of building a strong vibrant company with other organisations via speaking engagements and other opportunities.”
Simon Thompson, Key Group chief executive, said: “Dean has been an industry stalwart and an invaluable resource for the senior management team at Key Group. He has gradually been stepping back from being actively involved in the day to day running of the business and we wish him well as he focuses on consultancy work and speaking opportunities.”