There have been significant efforts to develop, adapt, and test interventions targeting the co-occurring mental health conditions common among autistic youth, with several extant narrative and systematic reviews demonstrating the efficacy of these interventions. Yet, a methodological analysis of the literature testing these interventions, particularly the characteristics of the participant samples comprising this evidence base, is needed to contextualize our current understanding of treatment effects and highlight current gaps in the current evidence base to inform future research. This systematic quantitative methodology review characterized participants included in the mental health treatment research for autistic youth. One hundred and thirty-one articles testing mental health interventions for autism spectrum disorder were coded for youth, caregiver, and provider participant demographics. Findings indicated limited representation of females, transition age youth, racial/ethnic minority groups, and youth diverse in cognitive functioning and co-occurring mental health symptoms or conditions. Limited inclusion of providers' representative of the clinical workforce was also noted. These results illuminate several critical gaps in the current evidence base for mental health interventions for autistic youth and provide recommendations for future research directions.