Importance: People with chronic mental illness (CMI) are at high risk of poor cardiorespiratory fitness as a result of sedentary behavior and physical inactivity. Occupational therapy practitioners play a key role as advocates for positive lifestyle change for people with CMI., Objective: To determine the relationships between occupational therapy activities and cardiorespiratory fitness among inpatients with CMI., Design: This retrospective research included three phases: descriptive cohort, case-control, and cross-sectional studies., Setting: Psychiatric inpatient facility., Participants: Inpatients with CMI, ages 18 to 65 yr (N = 325)., Outcomes and Measures: Data were collected over a 12-mo period. Each daily occupational therapy activity performed by participants was converted to energy expenditure (in kcal). Cardiorespiratory fitness was measured by means of the 3-Minute Step Test., Results: After daily occupational therapy activities, significantly more participants increased cardiorespiratory fitness than declined (McNemar χ2 [1] = 29.18, p < .05). Prevocational activities and moderate- to high-intensity exercises met the optimal energy expenditure level (>352 kcal) necessary to achieve an increase in cardiorespiratory fitness., Conclusions and Relevance: Occupational therapists in psychiatric inpatient settings should prescribe individualized occupation-based or physical activities that meet the optimal daily energy expenditure for each client to improve their cardiorespiratory function. What This Article Adds: This study is one of the first attempts to explore cardiorespiratory fitness outcomes after daily occupational therapy activities for people with CMI. Physical benefits unfolded throughout psychiatric care, echoing the profession's stance on holistic practice., (Copyright © 2022 by the American Occupational Therapy Association, Inc.)