Background: People with diagnoses of intellectual disability or serious mental illness have higher mortality rates due to physical comorbidities; better understanding is needed to guide best practice in provision of palliative care for these populations. Aims: To identify multivoiced perspectives, drawn from lived experience of: what works, and what does not, in palliative care for people with intellectual disability or serious mental illness; challenges in, and opportunities to improve, palliative care. Design: A systematically constructed qualitative meta-ethnography. Protocol published (PROSPERO: CRD42021236616). Data sources: MEDLINE, PsychINFO, CINAHL PLUS and Embase used without date limitations. Papers published in English, containing qualitative data on palliative care provision for people with a diagnosis of intellectual disability or serious mental illness were included. Global five-point strength score applied for relevance/quality appraisal. Results: Familiarity (of location, people and/or things) is important for good palliative care. Assumptions and misunderstandings about the role of mental capacity assessment to appropriately involve the patient in decision-making are common. Adapting training for palliative care staff to address concerns and beliefs about mental illness is one of the methods that helps avoid diagnostic overshadowing. Proactive identification of service arrangements to meet needs of persons with personality, psychotic, delusional and bipolar affective disorders will help optimise care. Conclusions: Evidence, including the voices of people with intellectual disability or serious mental illness is urgently needed to guide efforts to improve their access to and experience of palliative care. More evidence is especially needed to understand, develop and implement best practice for people with psychosis, bipolar affective disorder, mania and personality disorder.