The aim was to explore the utility of Keyes' concept of mental health in a substance addiction context. Mental health is considered the presence of emotional wellbeing in conjunction with high levels of social and psychological functioning. Using Keyes' measure, the frequency of languishing and flourishing is compared between clients who became abstinent and those continuing to use substances following treatment. It was hypothesised that there would be a significant interaction between substance use and levels of mental health over time. Participants were 794 individuals (79.5% male) attending residential substance abuse treatment provided by The Australian Salvation Army. The current sample was drawn from a larger longitudinal study evaluating routine client outcomes. At entry to treatment there were higher rates of languishing compared to population estimates, yet greater rates of flourishing at all time points compared to community normative data. There was a significant interaction between continuous mental health and substance use status. Mental health was rated significantly higher by individuals who were abstinent than those who had used substances at 3-month post-discharge follow-up. The comorbidity of mental illness and substance misuse has previously been investigated, but this is the first study to investigate the prevalence of mental health. While participants who remained abstinent achieved the highest levels of flourishing, at follow-up there were lower rates of languishing than found in a general community sample. Additionally, results suggested that improved mental health was a consequence of reduced severity of alcohol and other drug abuse, and followed reductions in cravings.