Community Sport Development Programs (CSDPs) that use an intersectoral capacity building approach have shown potential in reaching individuals in disadvantaged situations. This study has investigated how the application of capacity building principles in disadvantaged communities results in higher sport participation rates in these communities. A multiple case design was used, including six similar disadvantaged communities in Antwerp, Belgium; four communities implemented the CSDP, two communities served as control communities without CSDP. In total, 52 face-to-face interviews were held with sport, social, health, cultural, and youth organizations in these communities. Four key findings were crucial to explain the success of the CSDP according to the principles of capacity building. First, the CSDP appeared to be the missing link between sport organizations on the one hand and health, social, youth, and cultural organizations on the other hand. Second, shifting from a sport-oriented staff to a mix of sport staff, social workers and representatives of people in disadvantaged situations helped increase trust through a participatory approach. Third, CSDPs assisted sport clubs to deal with financial, organizational, and cultural pressures that arose from the influx of new members in disadvantaged situations. Finally, the CSDPs developed well-planned and integrated strategies focusing on reinforcing the existing local organizations already using sport to reach their goals. These capacity building principles were key in attaining higher sport participation for people living in disadvantaged communities.