Mass participation sport events are often cited as increasing population-level physical activity, but empirical evidence is scarce as to their effectiveness. The current research examined the developmental role of these events in increasing positive attitudes toward physically active leisure by a) determining its motivational capacity and b) investigating how sport event participation contributes to activity commitment and future exercise intentions. A multiattribute survey was distributed on-line to participants of a running event held in Philadelphia, Pennsylvania, (N = 2,791) three months after the event. Inferential statistics revealed that 10 motives directed participation across 75% of the population. Multivariate multiple linear regression analyses revealed event participation motives combined with event satisfaction and physical activity involvement explained 30% of running commitment and 31% of future exercise intentions. Most notably, the event promoted stronger attitudes toward regular exercise among participants who were more satisfied with their event experience, least active before the event, and had completed fewer prior organized events. This evidence demonstrates that mass participant sporting events can serve as important social and environmental correlates of physical activity.