Background: Children’s sedentary lifestyles and low physical activity levels may be countered using population-level interventions. This study examines factors influencing the use of a free community-wide physical activity access pass for grade 5 students (G5AP). Methods: A natural experiment with longitudinal data collection. A sample of 881 children completed the 9-month follow-up survey self-reporting where they used the G5AP. Two analyses were conducted: Getis-Ord GI* geographic cluster analysis of the spatial distribution of users, and logistic regression examining the relationship between use and accessibility (informational, economic, and geographic) and mobility options, while accounting for intrapersonal and interpersonal factors. Results: Overall, 44.9% of children used the G5AP with clusters of high use in urban areas and low use in the suburbs. Other factors significantly related to G5AP included gender (girls), informational accessibility (active recruitment), economic accessibility (median household income), geographic accessibility (facilities within 1.6 km of home), and mobility options (access to Boys & Girls Club bus). Conclusions: This study found that a diverse population of children used the G5AP. To continue being successful, community-based physical activity interventions need to ensure that the intervention increases geographic, economic, and informational accessibility and provides mobility options that are available to the target population.