According to several studies, the prevalence of Autism Spectrum Disorder (ASD) ranges from 2.4 to 9.9 percent among adult mental inpatients. However, subjects with forms of ASD that fit in the high functioning spectrum may remain undiagnosed during childhood and adolescence without reaching clinical attention until they develop in adult life other psychiatric disorders, often characterized by treatment resistance and poor outcomes. The aim of this case report was to evaluate the role of an undiagnosed ASD in the mental illness trajectory and discuss the diagnostic and therapeutic implications. We reported a case of a young man with an undiagnosed ASD that came to clinical attention only after the development of a severe manic episode with mixed and psychotic features and with catatonia in adulthood, despite meeting DSM-5-TR (APA, 2022) diagnostic criteria for ASD since early childhood. This case confirms the need of a timely identification of ASD in order to prevent the development of a mental illness trajectory and to improve the prognosis and the outcome. Moreover, on the heuristic level, this case seems to support the presence of a continuum between ASD and catatonia. In this framework, the use of a questionnaire based on a spectrum model, such as the AdAS Spectrum, could be useful in early diagnosis of ASD without intellectual or language impairment as well as in early detection of subthreshold conditions (broad autism spectrum phenotype or autistic traits), which represents a vulnerability factor for the development of various mental disorders.