We use a prevalence-based approach to estimate annual costs, in a wide-range of categories, associated with untreated mental illness (MI) in Indiana. Economic burden of untreated MI in Indiana is estimated at $4.2 billion annually representing 1.2% of the state’s gross domestic product. Considering average Indiana wages, $4.2 billion is equivalent to approximately 100,000 jobs. Cost of untreated MI includes $3.3 billion for indirect costs such as premature mortality, $708.5 million for direct healthcare costs, and $116.4 million for non-health care costs including incarceration. On average, each individual experiencing untreated MI incurs $18,940 of societal costs; much of which is borne by employers in the form of premature mortality, unemployment, absenteeism and presenteeism.