Background Youth with autism spectrum disorder (ASD) exhibit high rates of psychopathology. These symptoms can pose a risk of injury to self or others when the child is in crisis. Despite this danger, there are no instruments available to identify those with ASD who are at risk or actively in crisis. This study examined the psychometric properties of the Mental Health Crisis Assessment Scale (MCAS), a 28 item parent report measure. Methods The MCAS was administered to the parents of 606 children and young adults (aged 3–25 years, M age = 13 years, SD = 5 years) enrolled in the Interactive Autism Network, an online registry of families raising a child with ASD. The MCAS asks parents to rate the severity of various emotional and behavioral symptoms exhibited by their child. The parent then selects the behavior they perceive as the most dangerous behavior and rates the acuity of as well as their efficacy in managing this behavior. The MCAS was tested for internal consistency, construct validity, criterion validity, and convergent validity. Results The MCAS demonstrated strong internal consistency (Total Scale Cronbach's α = .88). The exploratory and confirmatory factor analyses suggested that a two factor (acuity and behavioral efficacy) model fit the data well, providing evidence of construct validity. Criterion validity, which was assessed by comparing the MCAS to clinician determination of crisis, indicated high levels of agreement (ROC = .85). Strong positive relationships emerged between the MCAS and measures of family distress (r = .56), parental stress, and frustration (r = .48), and use of emergency psychiatric services (OR = 24.2, 95% CI: 8.6–68.2), indicating convergent validity of the measure (all p < .05). Conclusions Results of the psychometric analyses suggest the MCAS appears to be a promising tool that can measure mental health crises in youth with ASD.