To examine the individual and combined associations of unhealthy behaviours (low/intermediate physical activity, consuming fruit and vegetables less than once a day, current smoking/short term ex-smoking, never/former/heavy alcohol drinking), assessed at start of follow-up, with hazard of disability among older French adults and to assess the role of potential mediators, assessed repeatedly, of these associations. Population based cohort study. Dijon centre of Three-City study. 3982 (2410 (60.5%) women) French community dwellers aged 65 or over included during 1999-2001; participants were disability-free at baseline when health behaviours were assessed. Hierarchical indicator of disability (no, light, moderate, severe) combining data from three disability scales (mobility, instrumental activities of daily living, basic activities of daily living) assessed five times between 2001 and 2012. During the 12 year follow-up, 1236 participants (861 (69.7%) women) developed moderate or severe disability. Interval censored survival analyses (adjusted for age, sex, marital status, and education) showed low/intermediate physical activity (hazard ratio 1.72, 95% confidence interval 1.48 to 2.00), consuming fruit and vegetables less than once a day (1.24, 1.10 to 1.41), and current smoking/short term ex-smoking (1.26, 1.05 to 1.50) to be independently associated with an increased hazard of disability, whereas no robust association with alcohol consumption was found. The hazard of disability increased progressively with the number of unhealthy behaviours independently associated with disability (P<0.001); participants with three unhealthy behaviours had a 2.53 (1.86 to 3.43)-fold increased hazard of disability compared with those with none. Reverse causation bias was verified by excluding participants who developed disability in the first four years of follow-up; these analyses on 890 disability events yielded results similar to those in the main analysis. 30.5% of the association between the unhealthy behaviours score and disability was explained by body mass index, cognitive function, depressive symptoms, trauma, chronic conditions, and cardiovascular disease and its risk factors; the main contributors were chronic conditions and, to a lesser extent, depressive symptoms, trauma, and body mass index. An unhealthy lifestyle is associated with greater hazard of incident disability, and the hazard increases progressively with the number of unhealthy behaviours. Chronic conditions, depressive symptoms, trauma, and body mass index partially explained this association.