Her resignation follows the publication earlier this week of a consultation document setting out a vision for CQC reform over the next three years.
Dame Jo said: ‘Having served on the board of the CQC for almost four years, and as chair for nearly three, I am proud of the progress we have made.
‘It has been a demanding and complex role, and there have undoubtedly been challenges as we registered 40,000 providers and brought the entire health and social care system under one set of standards.
‘But there is now clear evidence that our regulation is beginning to have an impact on the care that people receive, and it feels as if the organisation is moving into the next stage of its development.
‘This week, we have published a consultation document setting out proposals for CQC’s strategic direction for the next three years.
‘And I am delighted that that I have been able to appoint David Behan as CQC’s new chief executive - I am confident that he will continue to build on the progress that we have made in promoting and protecting the health and safety of people who use services.
‘It has been a privilege to hold this important role but I now believe it is time to step aside and for a new chair to lead CQC into the next stage.'
Commenting on Dame Jo's resignation, Sir Christopher Kelly, former permanent secretary of the DH said: ‘Over many years of devoted public service, Jo Williams has shown tireless commitment to improving care for individuals, carers and their families, in particular people who rely on mental health and learning disability services.
‘In all her roles she has shown great integrity and provided strong leadership. She has never shied away from taking on tough challenges. There are many who owe her a deep debt of gratitude.’
Dame Jo will remain in post until a successor is appointed.