A search committee is being put together to identify candidates for Nowak's position, and William Penn veteran Helen Davis Picher has been named interim president. She most recently served as director of evaluation and planning for William Penn.
Nowak said in a statement: "I am extraordinarily proud of the work we have done that will guide the William Penn Foundation for years to come. I am confident the foundation is well positioned to positively impact the region - particularly with so many facing such economic uncertainty. I look forward to continuing to support the foundation to address the critical needs of the greater Philadelphia region."
Before coming to William Penn, Nowak was the founding chief executive of The Reinvestment Fund, a well-known organization focused on urban development via housing, community arts centers, schools, commercial real estate, and sustainable energy projects.
Foundation officials said they remained committed to the strategic plan hammered out under Nowak.
In announcing Nowak's departure, members of the Haas family, which started the $2 billion philanthropy, emphasized continuing commitment to children, culture and the environment.
The new strategic plan, foundation officials stressed, endeavors to serve the community by closing the achievement gap for children from low-income families; ensuring a sustainable environment by protecting the Delaware and Susquehanna watersheds; and advancing Philadelphia as a leading center of creativity that enhances civic life.
These broad principles of the plan remain unchanged, officials said, but its "implementation continues to evolve."
The foundation will issue new program guidelines and application procedures in late January.
Contact culture writer Stephan Salisbury at 215-854-5594, firstname.lastname@example.org, or @SPSalisbury on Twitter.