Abstract Objectives The risks of developing cancer and dementia increase as we age; however, this comorbidity remains relatively under-researched. This study reports on the challenges that people affected by comorbid cancer and dementia face when navigating engagement with cancer treatment within secondary care. Materials and methods An ethnographic study recruiting 17 people with cancer and dementia, 22 relatives and 19 oncology staff in two UK National Health Service Trusts. Observations (46 h) and informal conversations were conducted during oncology appointments involving people with dementia. Semi-structured interviews (n = 37) with people living with cancer and dementia, their relatives and staff working in various roles across oncology services were also carried out. Data were analysed using ethnographically informed thematic analysis. Results People with cancer and dementia experienced challenges across three areas of navigating cancer treatment and care: navigating through multiple services, appointments and layers of often complex information; repeatedly navigating transport to and from hospital; and navigating non-dementia-friendly hospital outpatient environments alongside the cognitive problems associated with dementia. Conclusions Dementia impacts patients’ abilities to navigate the many practical aspects of attending hospital for cancer treatment and care. This study indicates the importance of addressing ways to improve the experience of travelling to and from the hospital, alongside extending the ongoing efforts to develop ‘dementia-friendly’ hospital in-patient areas and practices, to outpatient departments. Such steps will serve to improve hospital-based cancer treatment and care and more broadly outpatient appointment experiences for people with dementia and their families.