Community-based programmes are a widely implemented approach for population health promotion. Due to the context-dependent and dynamic nature of these programmes, evaluating their implementation is challenging. Identifying key events in the implementation process in evaluation could enable us to support future implementation, while acknowledging the complexity of real-world implementation. We studied the nationwide implementation of the Dutch Healthy Youth, Healthy Future (JOGG) approach, a community-based programme for childhood overweight prevention. The aims of our study were (i) to gain insights into the implementation process of the JOGG approach, and (ii) to identify key events that influenced said process. In nine communities, we conducted interviews (n = 24) with coordinators and stakeholders involved in the implementation of the JOGG approach and collected documents on the programme's implementation. We applied the analytical tool 'Critical Event Card' to identify key events in the implementation process. Results showed that in 5-10 years of implementing the JOGG approach, communities have undergone different phases: preparation, upscaling, resource mobilization, integration with other policy initiatives and adaptation of the implementation strategy. Key events influencing the implementation process included national policy developments (e.g. new health programmes), framing of the JOGG approach in local policy, staff turnover and coordination teams' experiences and actions. Furthermore, changes in implementation were often triggered by the destabilization of the implementation process and linked to opportunities for change in the policy process. The identified key events can inform future implementation of the JOGG approach as well as other community-based health promotion programmes.