This systematic review and meta-analysis investigated the benefits, safety and adherence of exercise interventions delivered in inpatient mental health settings, quantified the number of exercise trials that provided support to maintain engagement in exercise post-discharge, and reported patient feedback towards exercise interventions. Major databases were searched from inception to 22.06.2022 for intervention studies investigating exercise in mental health inpatient settings. Study quality was assessed using Cochrane and ROBINS-1 checklists. Fifty-six papers were included from 47 trials (including 34 RCTs), bias was high. Exercise improved depression (Standardised mean difference = −0.416; 95% Confidence interval −0.787 to −0.045, N = 15) compared to non-exercise comparators amongst people with a range of mental illnesses, with further (albeit limited) evidence suggesting a role of exercise in cardiorespiratory fitness and various other physical health parameters and ameliorating psychiatric symptoms. No serious exercise-related adverse events were noted, attendance was ≥80% in most trials, and exercise was perceived as enjoyable and useful. Five trials offered patients post-discharge support to continue exercise, with varying success. In conclusion, exercise interventions may have therapeutic benefits in inpatient mental health settings. More high-quality trials are needed to determine optimal parameters, and future research should investigate systems to support patients to maintain exercise engagement once discharged.