Importance Adults with severe mental illness (SMI) face a 10- to 20-year reduction in life expectancy, largely due to heightened cardiometabolic diseases. Low levels of physical activity (PA) and prolonged sedentary behavior (SB) are modifiable risk factors that contribute to this mortality gap. Although strong evidence demonstrates that PA is safe and effective in improving psychiatric and physical outcomes, systematic integration into clinical practice remains limited. Observations Individuals with SMI are among the most physically inactive groups in society, often spending more than 10 hours per day sedentary and rarely meeting recommended PA recommendations. Low PA and high SB exacerbate neuroinflammatory, neuroendocrine, and cardiometabolic pathways implicated in psychiatric morbidity. Recent meta-analyses show that structured PA, that is, exercise, produces moderate to large reductions in depressive and psychotic symptoms, as well as benefits for cognition, quality of life, and cardiometabolic health. Evidence also suggests that mentally passive SB, such as prolonged television viewing, are associated with poorer mental health outcomes. Physical activity should be individualized to each person’s capabilities and preferences, emphasizing forms they find enjoyable. Two sessions of strength training weekly are advised, and greater mental health benefits typically arise from leisure or active travel PA. To guide translation into care, the 5A framework (Ask, Assess, Advise, Assist, Arrange) provides a structured, pragmatic approach: clinicians can screen for PA and SB, assess readiness and safety, provide tailored advice, support motivation and goal setting, and arrange follow-up, referral, and community support. Conclusions and Relevance PA should be considered a core component of psychiatric care. By systematically promoting PA and reducing prolonged SB through structured clinical frameworks, psychiatrists and other mental health professionals can improve symptoms, enhance physical and cognitive health, and help narrow the life expectancy gap experienced by people with SMI.