News of Farrell’s unannounced departure from the post she has held since 2009 surprised early years sector leaders as it follows a major relaunch of the organisation in March.
Pacey confirmed Farrell left the charity on 31 July "to move on to a different stage in her career”.
The organisation also confirmed that Farrell’s former co-chief executive Liz Bayram has taken over as full-time chief executive of the charity.
A statement issued by Pacey said: “After almost eight years with the organisation, and having supported Pacey to develop its new strategy and emerge with a new brand, Catherine felt it was time to move on to a different stage in her career.
“She wishes Pacey and its members the very best for the future.”
United for All Ages and Good Care Guide director Denise Burke said Farrell’s departure was unexpected, “particularly after she has led such a significant change”.
“Despite my support for job-share, I’ve never believed a job-share at chief executive level works,” said Burke.
“In recent times Pacey’s members have criticised how it has provided support to childminders, especially with regard to childminding agencies,” she added.
In March, Pacey dropped the name and brand of the National Childminding Association (NCMA) which it had used since its formation in 1977.
Its relaunch marked the extension of its services to a wider range of childcare providers.
At the time, Farrell and Bayram issued a joint statement that said: “For the first time Pacey will enable one voice to represent all childcare professionals.”
Farrell joined Pacey as its director of finance in 2006 and became joint-chief executive in June 2009.
Bayram joined Pacey as director of policy and public affairs in 2004, and became chief executive in December 2005.
Pacey is currently campaigning against government proposals to introduce childminder agencies, which it believes threaten quality in childcare provision.