Evidence indicates that autism spectrum disorder (ASD) is underidentified in populations with psychosis, but that clinical presentations of comorbid ASD and psychosis (ASD-P) and specific treatment needs that may relate to this group are not well understood. In fact, recent studies of ASD in first-episode psychosis suggest that there may be a specific clinical presentation of ASD-P. In response, the Cambridgeshire and Peterborough Early Intervention in Psychosis (EIP) service in the UK implemented and evaluated a 3-step ASD screening and diagnostic protocol, using the Autism Spectrum Disorder in Adults Screening Questionnaire (ASDASQ), case note review, and the Autism Diagnostic Observation Schedule (ADOS-2) and the Childhood Autism Spectrum Test (CAST). As a quality improvement project, the evaluation aimed to (1) establish the prevalence of patients with ASD-P, (2) describe characteristics of the clinical presentation of ASD-P and compare them to those of patients suffering from psychosis but no ASD, and (3) determine any differences in treatment between psychosis patients with and without ASD. Notably, at least 9% of the EIP service caseload met the criteria for a diagnosis of ASD-P, with half identified via the implementation of this protocol. The patients with ASD-P had specific clinical presentations and treatment needs that differed from those of patients with psychosis but no ASD. Thus, the findings from this study supported existing evidence concerning the underdetection of ASD in EIP populations. Our findings also added to emerging evidence for a clinical presentation of ASD-P with specific treatment needs. Our protocol has now been established as routine practice, and its implementation has improved the detection and treatment of patients with ASD-P within our EIP service.