Mega sport events are frequently criticized for excessive commercialization and sustainability concerns. By contrast, non-mega sport events (NMSEs) offer more cost-effective, locally suitable, and socially sustainable benefits. To understand the lasting influence of NMSEs on local sport participation, we constructed a research model based on systems theory and integrated with youth and grassroots sport development legacies into a trickle-down effect model to examine their mediating roles. In the context of the National Sports Festival, an annual Japanese sport competition, we surveyed sport and government officials (n = 158) on the legacies of youth and grassroots sport development, sport infrastructure, sport inspiration, and sport participation. The findings indicate both sport infrastructure and sport inspiration legacies have positive indirect effects on sport participation legacy through grassroots sport development legacy. This study adds new insights on the importance of NMSEs, identifying grassroots sport development legacy as a mediating mechanism that facilitates sport participation.