The organizational social context in which mental health services are provided is believed to affect the adoption and implementation of evidence-based practices (EBPs) as well as the quality and outcomes of the services. A fully developed science of implementation effectiveness requires conceptual models that include organizational social context and tools for assessing social context that have been tested in a broad cross-section of mental health systems. This paper describes the role of organizational social context in services and implementation research and evaluates a comprehensive contextual measure, labeled Organizational Social Context (OSC), designed to assess the key latent constructs of culture, climate and work attitudes. The psychometric properties of the OSC measure were assessed in a nationwide study of 1,154 clinicians in 100 mental health clinics with a second-order confirmatory factor analysis of clinician responses, estimates of scale reliabilities, and indices of within-clinic agreement and between-clinic differences among clinicians. Finally, the paper illustrates the use of nationwide norms in describing the OSC profiles of individual mental health clinics and examines the cross-level association of organizational-level culture and climate with clinician-level work attitudes.