The present study addressed the different contributors to social anxiety in children with Selective Mutism (SM), with and without co-occurring Autism Spectrum Disorder (ASD) (SM + ASD). Seventy-five parents completed an online composite questionnaire consisting of the symptoms of SM and ASD, anxiety and sensory measures. The results found the SM + ASD group showed significantly higher levels of social anxiety and sensory avoidance compared to the SM only group. However, a simple mediation model revealed sensory avoidance to be a mediator of this relationship between the diagnosis and social anxiety. Therefore, higher levels of sensory avoidance may help to differentiate social anxiety between the groups and may also be a sign of ASD in children with SM who have and/or are yet to receive an ASD diagnosis.