Adults with autism spectrum disorder (ASD) experience significant impairments in social interaction, poor planning, decision making, timing and motor skills which impact on their daily activities of living and ability to access the health care services. Early research on ASD primarily focused on children with service provision for adults is still in its infancy. The current treatment for adults with ASD is complex, considering the diverse etiology and characteristics. In this study, a 25-year-old male diagnosed with ASD at the age of 7 years underwent intrathecal autologous bone marrow derived mononuclear cells (BMMNCs) administration, followed by neurorehabilitation. Following six months of cell therapy, the patient showed improvement in concentration, attention, command following, sitting tolerance, social interactions, eye contact, and memory. His Childhood Autism Rating Scale (CARS), Indian Scale of Assessment of Autism (ISAA), Functional Independence Measure (FIM) scores improved from 36 to 34, 87 to 78, and 65 to 78, respectively. As compared to pre-intervention, brain ¹⁸F-FDG PET/CT scan showed improvement in supramarginal gyrus, thalamus, basal ganglia, medial temporal cortex and cerebellum. This study suggests the efficacy of autologous BMMNCs in addressing the core neurodeficits of adult ASD, and thereby improving the quality of life.