Objectives: To assess physical activity (PA) and sedentary behaviour (SB) in non-institutionalised adults with mental illness, using a combination of self-report and objective measures. Design: Cross-sectional Methods: Participants completed PA questionnaires (time spent walking for transport, walking for recreation, gardening, vigorous-, and moderate-intensity activities), and SB questionnaires (time spent sitting for TV, travel, work, computer use, and reclining). Participants also wore an accelerometer for 7 days. Accelerometry estimates of time spent in SB, light activity, and moderate-to-vigorous activity (MVPA), bout durations, and, breaks in sedentary time, were calculated. Results: 142 participants completed the questionnaires. The median time spent in self-reported MVPA and SB was 4.5 h/week and 10.7 h/day, respectively. Walking for transport, and sitting to watch TV, contributed most to self-report estimates; time spent reclining was an important contributor to SB. Ninety-nine participants completed the accelerometry. The median time spent in accelerometer-derived MVPA and SB was 26 min/day and 9.2 h/day respectively; 7% of MVPA time was in bouts of 10 min or more, and 34% of SB time was in bouts of over 20 min. Conclusions: A high proportion of participants reported activity levels consistent with physical activity guidelines; however, a small proportion of activity was accumulated in bouts of 10 min or more. Participants also had high levels of SB, about one-third of which was accumulated in bouts over 20 min. PA and SB interventions for this group could target increasing recreational walking, and reducing television time. (C) 2015 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved.