Abstract Background Providing cancer care and treatment for ageing populations with complicating comorbidities like dementia is a growing global challenge. This study aimed to examine the hospital-based cancer care and treatment challenges and support needs of people with dementia, and identify potential ways to address these. Methods A two-site ethnographic study in England involving semi-structured interviews, observations and accompanying conversations, and medical record review. Participants (N = 58) were people with dementia and comorbid cancer (n = 17), informal caregivers (n = 22) and hospital staff (n = 19). Ethnographically informed thematic analysis was conducted. Results There was an accumulated complexity of living with both illnesses simultaneously. People with dementia and families could feel confused and uninformed due to difficulties understanding, retaining and using cancer information, which impacted their informed treatment decision-making. Dementia increased the complexity and burden of travelling to and navigating unfamiliar hospital environments, frequent lengthy periods of waiting in hospital, and self-managing symptoms and side-effects at home. Oncology staff were often working without the full picture, due to variable documenting of dementia in medical records, dementia training was limited, and time and resource pressures impeded the highly individualised, flexible cancer care required by people with dementia. Supportive family carers were crucial in enabling people with dementia to access, navigate and undergo cancer treatment and care. Conclusions Dementia complicates cancer care in a range of ways accumulating across the cancer pathway. Our findings suggest there are several strategies and interventions, which we list here, with potential to improve cancer care and treatment for people with dementia and their families.