Background: The aim of this study was to assess the feasibility of using questionnaires and accelerometers to measure physical activity and sedentary behavior among inpatient adults with mental illness. Methods: Participants completed a physical activity and sitting time questionnaire and wore an accelerometer for 7 consecutive days. Feasibility was assessed in terms of participant engagement, self-reported ease/difficulty of completing study components, extreme self-report data values and adherence to accelerometer wear time criteria. Ease/difficulty ratings were examined by level of distress. Results: 177 inpatients were invited to the study, 101 completed the questionnaires and 36 provided valid accelerometry data. Participants found it more difficult to complete sitting time and physical activity questionnaires than to wear the accelerometer during waking hours (z = 3.787, P < .001; z = 2.824, P = .005 respectively). No significant differences were found in ease/difficulty ratings by level of distress for any of the study components. Extreme values for self-reported sitting time were identified in 27% of participants. Conclusion: Inpatient adults with mental illness can engage with self-report and objective methods of measuring physical activity and sedentary behavior. They were initially less willing to participate in objective measurement, which may however be more feasible than self-report measures.