Developed in collaboration with WHO Department of Mental Health and Substance Abuse, this study (conducted in India, the UK, and the USA) integrated feedback from mental health service users into the development of the chapter on mental, behavioural, and neurodevelopmental disorders for ICD-11. The ICD-11 will be used for health reporting from January, 2022. As a reporting standard and diagnostic classification system, ICD-11 will be highly influential by informing policy, clinical practice, and research that affect mental health service users. We report here the first study to systematically seek and collate service user perspectives on a major classification and diagnostic guideline. Focus groups were used to collect feedback on five diagnoses: depressive episode, generalised anxiety disorder, schizophrenia, bipolar type 1 disorder, and personality disorder. Participants were given the official draft diagnostic guidelines and a parallel lay translation. Data were then thematically analysed, forming the basis of co-produced recommendations for WHO, which included features that could be added or revised to better reflect lived experience and changes to language that was confusing or objectionable to service users. The findings indicated that an accessible lay language version of the ICD-11 could be beneficial for service users and their supporters.