Objective To study the stability and emergence of a range of behavioural and psychological symptoms (BPS), their association with mortality and the effect of covariates on these transitions in a population‐based study of cognitively impaired older people with a long follow‐up period and large sample size, with a particular focus on apathy. Methods Data were from a population‐based, longitudinal cohort study of ageing. Interviews were conducted at 0, 2, 6, 8 and 10 years with 3626 participants aged 65+. The persistence of 11 BPS and their association with mortality in those with cognitive impairment (MMSE 25 or below) was investigated using multi‐state models, allowing us to take into account estimations of the probability of transitions that occurred in the time between interviews. Results Most BPS were persistent. Apathy was one of the most stable symptoms; in those with apathy, the probability of still having apathy after 1 year is 62%. Apathy, sleep problems, depression, irritability and wandering were most likely to develop. BPS are associated with mortality; in those with apathy, mortality is 3.1 times more likely than in those without apathy. Low cognitive function and dementia were associated with emergence of new symptoms. Conclusions This population‐based, multi‐centre study with a follow‐up period of 10 years showed that BPS are associated with mortality and most symptoms are persistent. Apathy was characterised by a high prevalence, a high persistence and a strong association with mortality, and has a negative impact on disability, management of other disease and caregiver burden.