Developed in collaboration with the WHO Department of Mental Health and Substance Abuse, this study conducted in the UK, India, and the US, integrated feedback from mental health service users into the development of the chapter on mental, behavioural, and neurodevelopmental disorders for the Eleventh Revision of the International Classification of Diseases and Related Health Problems (ICD-11). The ICD-11 is set for approval by the World Health Assembly in May, 2019. As a reporting standard and diagnostic classification system it will be highly influential on the policy, clinical practice, and research that affect mental health service users; yet this is 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 thematically analysed. This formed the basis of co-produced recommendations for the 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 also indicated that an accessible lay language version of the ICD-11 could be beneficial for service users and their supporters.