Objectives: The purpose of this study was to determine if social cohesion mediates the effects of neighborhood and household-level socioeconomic status (SES), perceptions of neighborhood safety, and access to parks on mothers' engagement in physical activity (PA). Method: Secondary analyses were conducted on cross-sectional data from The Geographic Research on Wellbeing (GROW) study. GROW includes survey data from a diverse sample of 2,750 California mothers. Structural equation modeling was used to test a conceptual multilevel mediation model, proposing social cohesion as a mediator of known predictors of PA. Results: Social cohesion fully mediated the pathway from perceived neighborhood safety to mothers' PA. Social cohesion also mediated the significant relationship between neighborhood SES and PA; however, this mediation finding was not practically significant when considered in the context of the full model. Household SES was significantly positively related to both social cohesion and PA. Park access contributed significantly to social cohesion but not directly to PA Social cohesion did not significantly mediate relationships between park access or household SES and PA. Conclusions: There is a need for public health interventions to improve engagement in PA among individuals and neighborhoods with lower levels of socioeconomic resources. Interventions that create social cohesion within neighborhoods may have positive effects on mothers' PA, particularly in neighborhoods perceived as unsafe.