Background: The drivers of costs of care for people with dementia are not well understood and little is known on the costs of care for those with rarer dementias. Objective: To characterize use and costs of paid and unpaid care over time in a cohort of people with dementia living in Britain. To explore the relationship between cohort members’ demographic and clinical characteristics and service costs. Methods: We calculated costs of health and social services, unpaid care, and out-of-pocket expenditure for people with mild-to-moderate dementia participating in three waves of the IDEAL cohort (2014–2018). Latent growth curve modelling investigated associations between participants’ baseline sociodemographic and diagnostic characteristics and mean weekly service costs. Results: Data were available on use of paid and unpaid care by 1,537 community-dwelling participants with dementia at Wave 1, 1,199 at Wave 2, and 910 at Wave 3. In models of paid service costs, being female was associated with lower baseline costs and living alone was associated with higher baseline costs. Dementia subtype and caregiver status were associated with variations in baseline costs and the rate of change in costs, which was additionally influenced by age. Conclusion: Lewy body and Parkinson’s disease dementias were associated with higher service costs at the outset, and Lewy body and frontotemporal dementias with more steeply increasing costs overall, than Alzheimer’s disease. Planners of dementia services should consider the needs of people with these relatively rare dementia subtypes as they may require more resources than people with more prevalent subtypes.