A continuous assessment and a categorical diagnosis of the presence (i.e., flourishing) and the absence (i.e., languishing) of mental health were proposed and applied to the Midlife in the United States study data, a nationally representative sample of adults between the ages of 25 and 74 years (N = 3,032). Confirmatory factor analyses supported the hypothesis that measures of mental health (i.e., emotional, psychological, and social well-being) and mental illness (i.e., major depressive episode, generalized anxiety, panic disorder, and alcohol dependence) constitute separate correlated unipolar dimensions. The categorical diagnosis yielded an estimate of 18.0% flourishing and, when cross-tabulated with the mental disorders, an estimate of 16.6% with complete mental health. Completely mentally healthy adults reported the fewest health limitations of activities of daily living, the fewest missed days of work, the fewest half-day work cutbacks, and the healthiest psychosocial functioning (low helplessness, clear life goals, high resilience, and high intimacy).