Abstract Background In 2018, the Dutch government initiated the Solid Start program to provide each child the best start in life. The program focuses on the crucial first thousand days of life, which span from preconception to a child’s second birthday, and has a specific focus towards (future) parents and young children in vulnerable situations. A key program element is improving collaboration between the medical and social sector by creating Solid Start coalitions. This study aimed to describe the implementation of the Dutch Solid Start program, in order to learn for future practice and policy. Specifically, this paper describes to what extent Solid Start is implemented within municipalities and outlines stakeholders’ experiences with the implementation of Solid Start and the associated cross-sectoral collaboration. Methods Quantitative and qualitative data were collected from 2019 until 2021. Questionnaires were sent to all 352 Dutch municipalities and analyzed using descriptive statistics. Qualitative data were obtained through focus group discussions(n = 6) and semi-structured interviews(n = 19) with representatives of care and support organizations, knowledge institutes and professional associations, Solid Start project leaders, advisors, municipal officials, researchers, clients and experts-by-experience. Qualitative data were analyzed using the Rainbow Model of Integrated Care. Results Findings indicated progress in the development of Solid Start coalitions(n = 40 in 2019, n = 140 in 2021), and an increase in cross-sectoral collaboration. According to the stakeholders, initiating Solid Start increased the sense of urgency concerning the importance of the first thousand days and stimulated professionals from various backgrounds to get to know each other, resulting in more collaborative agreements on cross-sectoral care provision. Important elements mentioned for effective collaboration within coalitions were an active coordinator as driving force, and a shared societal goal. However, stakeholders ...