Autism assessments for children who are deaf are particularly complex for a number of reasons, including overlapping cultural and clinical factors. We capture this in an ethnographic study of National Health Service child and adolescent mental health services in the United Kingdom, drawing on theoretical perspectives from transcultural psychiatry, which help to understand these services as a cultural system. Our objective was to analyse how mental health services interact with Deaf culture, as a source of cultural-linguistic identity. We ground the study in the practices and perceptions of 16 professionals, who have conducted autism assessments for deaf children aged 0–18. We adopt a framework of intersectionality to capture the multiple, mutually enforcing factors involved in this diagnostic process. We observed that professionals working in specialist Deaf services, or with experience working with the Deaf community, had intersectional understandings of assessments: the ways in which cultural, linguistic, sensory, and social factors work together to produce diagnoses. Working with a diagnostic system that focuses heavily on ‘norms’ based on populations from a hearing culture was a key source of frustration for professionals. We conclude that recognising the intersectionality of mental health and Deaf culture helps professionals provide sensitive diagnoses that acknowledge the multiplicity of D/deaf experiences.