Background: International cardiac rehabilitation guidelines recommend that participants meet public health physical activity guidelines. Few studies have objectively measured how much time cardiac rehabilitation participants spend in physical activity and sedentary behaviour, particularly over the long term. Objective: The aim of this study was to objectively assess physical activity and sedentary behaviour of cardiac rehabilitation participants over 12 months and determine whether they met the public health physical activity and sedentary behaviour guidelines. Methods: Cardiac rehabilitation participants with coronary heart disease were recruited in a prospective cohort study (n = 72). Participants wore an ActiGraph ActiSleep accelerometer for 7 consecutive days at baseline, 6 weeks, and 6 and 12 months to assess daily minutes of moderate-to-vigorous physical activity and sedentary behaviour (< 100 counts/min). Other outcomes collected were self-reported physical activity and sedentary behaviour, body mass index, waist-to-hip ratio, lipid profile, blood glucose level, quality of life, exercise capacity, anxiety and depression. Results: By intent-to-treat analysis, during the 6-week cardiac rehabilitation program, participants increased their light physical activity (P < 0.01), which was maintained up to 12 months. Moderate-to-vigorous physical activity and sedentary behaviour did not change during the 6-week cardiac rehabilitation program but did improve over 6 months (sedentary behaviour decreased [P < 0.001], moderate-to-vigorous physical activity increased [P < 0.05]), which was maintained up to 1 year. Completion of moderate-to-vigorous physical activity in 10-min bouts did not change over 12 months, nor did the proportion of participants meeting physical activity guidelines (15–21%). Sedentary behaviour remained high throughout (11 hr/day). Conclusion: Most cardiac rehabilitation participants did not meet the physical activity guidelines during and after a 6-week program up to 12 months. Reducing sedentary behaviour may be a more achievable first-line strategy for cardiac patients, moving participants along the energy expenditure continuum, aiming to increase their physical activity levels over the medium to long term. Trial registration: Australian New Zealand Clinical Trials Registry (ANZCTR): ACTRN12615000995572, http://www.ANZCTR.org.au/ACTRN12615000995572.aspx.