Medical assistance in dying, which includes voluntary euthanasia and assisted suicide, is legally permissible in a number of jurisdictions, including the Netherlands, Belgium, Switzerland and Canada. Although medical assistance in dying is most commonly provided for suffering associated with terminal somatic illness, some jurisdictions have also offered it for severe and irremediable psychiatric illness. Meanwhile, recent work in the philosophy of psychiatry has led to a renewed understanding of psychiatric illness that emphasises the role of the relation between the person and the external environment in the constitution of mental disorder. In this paper, I argue that this externalist approach to mental disorder highlights an ethical challenge to the practice of medical assistance in dying for psychiatric illness. At the level of the clinical assessment, externalism draws attention to potential social and environmental interventions that might have otherwise been overlooked by the standard approach to mental disorder, which may confound the judgement that there is no further reasonable alternative that could alleviate the person's suffering. At the level of the wider society, externalism underscores how social prejudices and structural barriers that contribute to psychiatric illness constrain the affordances available to people and result in them seeking medical assistance in dying when they otherwise might not have had under better social conditions.