Nature-based physical activity programming (e.g., countryside walks, hiking, horseback riding) has been found to be an effective way to help improve the health of people with mental illness. Exercise referral initiatives, whereby health practitioners prescribe exercise in an attempt to prevent or treat chronic illnesses, have helped make such nature-based activities accessible to this population in the United Kingdom and Australia; however, there is a dearth of research related to the most prominent exercise referral program in the United States: Exercise is Medicine. Taking into account the barriers to physical activity faced by people with mental illness, we explore how nature-based programming for this population might be mobilized in the United States through the growing Exercise is Medicine initiative.