The objectives of this study were to estimate the prevalence of autism spectrum disorders (ASD) and intellectual disability (ID) among youths active in at least one of five public service systems: mental health (MH), educational services for youth with serious emotional disturbance (SED), child welfare (CW), juvenile justice (JJ), and alcohol and drug services (AD). This study also reports the characteristics and patterns of system involvement among these youths. Results indicate that approximately 12% of a random sample of youths involved in these public service systems had ID or ASD. These disabilities were particularly prevalent in youths in the SED (25%), MH (13%), and CW (13%) systems and were less prevalent in the JJ and AD systems (4% each). Youths with ID or ASD were more likely than other youths to be White, have a higher socioeconomic status, and be more likely to have externalizing psychiatric and other problems. Of those with ASD or ID, approximately one-third were served in more than one service system, with the MH and SED systems most likely to be serving youths with externalizing psychiatric disorders. These findings have important implications for service provision, treatment planning, and workforce development. (Contains 4 tables.)