Abstract Background Deficits in mentalizing and attachment occur in the autism and schizophrenia spectrum, and their extended traits in the general population. Parental attachment and the broader social environment highly influence the development of mentalizing. Given the similarities in the symptomatology and neurodevelopmental correlates of autism spectrum disorder (ASD) and schizophrenia (SCH), it is crucial to identify their overlaps and differences to support screening, differential diagnosis, and intervention. Methods This cross-sectional study utilized questionnaire data from 2203 adults (65.1% female, mean age[SD] = 37.98[9.66]), including participants diagnosed with ASD, SCH, and those exhibiting subclinical traits to investigate the associations between mentalizing, attachment, and perceived social support during adolescence across the autistic and schizotypy spectrum. Results It was revealed that both autistic and schizotypal traits have comparable effects on insecure adult attachment, primarily through challenges in mentalizing. The impact of mentalizing deficits on adult attachment slightly varies between autistic and schizotypal traits. Conversely, perceived social support during adolescence relates to improved mentalizing and secure adult attachment as a protective factor during development. Conclusions These outcomes highlight the significance of supportive therapeutic relationships and community care while suggesting directions for further research and collaborative treatments addressing ASD and SCH, considering the differential impact of mentalizing on adult attachment.