Evidence supporting the effectiveness of a developmental-focused youth sport (DYS) program designed exclusively for elementary school aged girls is mounting. The purpose of this study was to evaluate the impact of on the Girls on the Run program on psychological and physical assets among 3rd- to 5th-grade girls. A longitudinal quasi-experimental study was conducted to evaluate intervention effects among 877 participants categorized into 1 of 3 groups (never, newly, and previously exposed). A 64-item self-report survey measured developmental assets at 3 time-points. Nested random effects ANOVA models were used to compare demographic factors and psychological and physical assets between exposure groups and to compare longitudinal differences in these assets. After adjustment for multiple comparisons, previous program participants had significantly higher physical activity commitment (P = .006) and physical activity levels (P = .047) at preintervention than never exposed. From pre- to postintervention body image improved in newly exposed participants (P = .03). Physical activity increased from preintervention to follow-up among never and newly exposed participants (all P < .05). Although we were unable to fully confirm the study hypotheses, the results of the current study provide new evidence to support future long-term studies examining the effectiveness of an innovative DYS program for 3rd- to 5th-grade girls.