OBJECTIVES: To build upon state-of-the-art theory and empirical data to estimate the strength of multiple mediators of the efficacious Keep Active Minnesota (KAM) physical activity (PA) maintenance intervention. METHODS: The total, direct, and indirect effects through which KAM helped randomized participants (KAM n = 523; UC n = 526) maintain moderate or vigorous PA (MVPA) for up to 2 years were estimated using structural equation modeling. RESULTS: Multiple mediators explained half (beta = .052, P = .13) of the effect of KAM on MVPA (beta = .105, P = .004). Self-efficacy was the upstream variable in 2 endogenously mediated effects, and the self-concept mediator emerged as the strongest predictor of MVPA. CONCLUSIONS: KAM positively impacted self-efficacy, which was associated with PA enjoyment, integration into the self-concept, and PA maintenance. Successful long-term PA maintenance appears to be influenced by multiple small interrelated mediational pathways. Future research evaluating maintenance models should specify recursive relationships among mediators and outcomes.