To understand the development of inter-sectoral participation in the three intervention municipalities of Stockholm Diabetes Prevention Programme (SDPP) case studies with a longitudinal assessment were conducted using the spidergram method, document analysis and group discussions. At three time points, the members of the local steering committees assessed the extent of participation from narrow to wide inter-sectoral participation in five key areas: planning, resources, leadership, network and implementation. Wide participation of various interest groups was recognised in planning and implementing activities whereas local resources, the representation of the leadership and the extent of the network were perceived as more restricted. Expert involvement varied during the programme period but was not regarded as exerting control over the local programmes. Participation within the local steering committees decreased, with a stronger focus on the project co-ordinator and other local partners in latter years. The extent of partner engagement increased due to focusing on activities approaching multi-sector collaboration and institutionalisation. Overall, communication and shared responsibility appeared critical in influencing both the development and perception of participation. In conclusion, to understand the dynamic process of participation at different times, areas and levels, the development and use of evaluation designs combining different methods and information sources throughout the lifespan of a project are recommended.