Background: Social network–driven approaches have promise for promoting physical activity in community settings. Yet, there have been few direct investigations of such interventions. This study tested the effectiveness of a social network–driven, group-based walking intervention in a medically underserved community. Methods: This study used a quasi-experimental pretest–posttest design with 3 measurement time points to examine the effectiveness of Sumter County on the Move! in communities in Sumter County, SC. A total of 293 individuals participated in 59 walking groups formed from existing social networks. Participants were 86% females, 67% black, and 31% white, with a mean age of 49.5 years. Measures included perceptions of the walking groups; psychosocial factors such as self-regulation, self-efficacy, and social support; and both self-reported and objectively measured physical activity. Results: The intervention produced significant increases in goal setting and social support for physical activity from multiple sources, and these intervention effects were sustained through the final measurement point 6 months after completion of the intervention. Nonetheless, few of the desired changes in physical activity were observed. Conclusion: Our mixed results underscore the importance of future research to better understand the dose and duration of intervention implementation required to effect and sustain behavior change.