Most research on mental health among adolescents and young adults concentrates on understanding mental illness. However, mental health is more than the absence of mental illness. Among adolescents and young adults, positive mental health—a combination of emotional, social and psychological well-being—is related to higher prosocial behaviour, school integration and self-concept. However, much of the research on positive mental health among young adults has been with college students. Limited research has examined the presence and correlates of positive mental health, or flourishing, among a nationally representative sample of US young adults. This study extended from another researcher’s original examination of positive mental health among US adolescents to describe the prevalence of flourishing among these same individuals in young adulthood. Our sample included 1090 individuals from the 2011 Panel Study of Income Dynamics Transition into Adulthood Supplement. Univariate and bivariate tests were used to describe the prevalence of flourishing during young adulthood and changes from adolescence to young adulthood. We used multivariable logistic regression to examine the relationships among indicators of healthy development and flourishing. Results suggest that flourishing improved during the transition into young adulthood and that targeting factors like life skills and civic engagement may enhance flourishing.