Following development of a 46-item of measure citizenship, a framework for supporting the full membership in society of persons with mental illness, this study tested the measure’s reliability and validity. 110 persons from a mental health center completed a questionnaire packet containing the citizenship measure and other measures to assess internal consistency and validity of the citizenship instrument. Correlation matrices were examined for associations between the citizenship instrument and other measures. Stepwise regression examines demographic factors, sense of community, and social capital as predictors of citizenship, recovery, and well-being. Analyses revealed that the measure is psychometrically sound. The measure captures subjective information about the degree to which individuals experience rights, sense of belonging, and other factors associated with community membership that have been previously difficult to assess. The measure establishes a platform for interventions to support the full participation in society of persons with mental illnesses.