Synchrony is a multi-faceted construct, defined here as the mutual warmth and responsiveness between a mother and her child. As children with Autism Spectrum Disorders (ASD) struggle to navigate various aspects of social life, we expected that impairments in synchrony would be seen, but that mothers would show adaptability. Twenty-five children (10 met ASD criteria on a gold standard autism assessment) completed a brief cognitive assessment and nine semi-structured play tasks with their mother, adapted from the Marschak Interaction Method. Synchrony was not found to moderate the relationship between ASD severity and externalizing behaviors, as hypothesized. ASD severity did predict externalizing behaviors. While children in the ASD group were more negative during the observed play tasks, there were no group differences on mother or dyad scores. Despite power limitations, our findings suggest important future directions for examination of mother and dyad mechanisms that better explain these differences.