Up to 70% of children with autism spectrum disorders (ASD) have a co-occurring mental health disorder; however, many clinicians feel unprepared to serve children with complex co-occurring conditions. This study surveyed 64 mental health clinicians working in 21 publically-funded mental health agencies in a large urban setting to explore their preparation and supports for providing mental health services to children with ASD and other developmental disabilities. Results indicated that only half of clinicians received training in ASD/DD in their professional education, with social workers least likely to have had such training. Only half of clinicians felt confident to diagnose mental health problems or provide interventions to children with ASD/DD; those with training on ASD/DD during their professional education were more likely to report confidence. Agency supports were lacking, with just 16% of therapists having supervisors with expertise in ASD/DD, and only 23% receiving training in ASD/DD at their agency. Participants from county agencies were less likely to have agency supports for their work in ASD/DD compared to participants from private non-profit agencies. Implications are discussed for enhancing education and training to meet the mental health needs of children with ASD/DD.