Learning Care Group made a three-year commitment with the Partnership for a Healthier America to implement several evidence-based nutrition and physical activity policies in over 900 child care centers. New practices included serving more fruits and vegetables; eliminating sugary drinks and juice; family-style dining; healthier celebrations; limiting screen time; increasing outdoor play time; and supporting breastfeeding. A sequential convergent mixed-methods design was used to describe changes in center practices over time; organizational support for changes; and center director perceptions of the implementation process. Data were collected through an interview with organizational leadership; document review; and online center director surveys at 6, 18, and 36 months. The final center director survey included open-ended questions about policy implementation. Written responses were coded and overall themes were extracted by integrating the qualitative and quantitative data. The five overall themes were to: take a comprehensive approach; build the initiative over time; provide structural supports; replace old practices with new ones; and communicate thoroughly. Center leaders reported primarily positive reactions to healthier menus, juice removal, and increased physical activity. The most controversial policy concerned healthier celebrations. Center directors reported that the staff and children adapted quickly to changes, while some families required more assistance. The experience of this large child care organization can inform other efforts to implement early care and education wellness policies. (author abstract)