Childhood-Onset Psychosis (COP) is an extremely rare and severe variant of psychosis. This paper presents the largest UK case series of COP, comprising 39 families (105 individuals) recruited between 2016 and 2019. It is also the most ethnically diverse collection of COP cases worldwide, with 36% European ancestry, 23% South Asian, 18% African, 10% Mixed ancestry and 13% Other. All probands were hospitalised and diagnosed with a psychotic disorder by age 13, including schizoaffective disorder, paranoid schizophrenia, childhood-onset schizophrenia and other nonorganic psychotic disorders. The cohort had a male-to-female ratio of 1:1.4, with females diagnosed at a higher mean age (11 years 9 months) than males (10 years 6 months). While 54% of probands had no reported first-degree family history of mental disorders, 59% presented with comorbid neurodevelopmental conditions. Approximately one-third were treated with clozapine, reflecting the severity and treatment resistance of the cases.