BACKGROUND: Active mobility and public transport increase physical activity (PA) levels. With varying intensity and effectiveness, European cities implement Sustainable Urban Mobility Plans (SUMPs) to spur transport-related PA. Therefore, we aim to examine drivers and barriers to SUMP implementation and assess its influence on PA across European cities. METHODS: We screened policy reports to gain insights into SUMP implementation in one Danish, two German and two Polish cities. Further, we conducted semi-structured interviews with SUMP stakeholders in these cities to explore their experiences with SUMP implementation. Thematic analysis of interview transcripts was applied to identify similarities and differences across cities. To assess the effect of SUMP implementation on PA, we searched for data on indicators of transport-related PA. RESULTS: All investigated cities are committed to sustainable mobility. Nonetheless, complex institutional structures, the dominant role of motorized traffic as well as complex regional and local policy integration hamper SUMP implementation. Danish, German and Polish cities face different contexts in terms of financing, national guidelines and the prominence of sustainability as a policy objective. Each city adopts unique indicators for monitoring the effects of SUMPs on transport-related PA. The variety of indicators and limited data availability impede a comparative evaluation across cities. Constrained by this restriction, we identified motorization rate, modal split and public transport ridership as suitable indicators. CONCLUSIONS: Local idiosyncrasies need to be accounted for when assessing the implementation of SUMPs. Nonetheless, consistent indicators and data transparency are essential for comparing the effectiveness of SUMPs and their impact on PA.