Despite a rise in the observed prevalence of autism spectrum disorder among children, few estimates exist of the share of US outpatient mental health treatment facilities that provide services for children with autism spectrum disorder. We identified key facility- and county-level characteristics in offering mental health care for children with autism spectrum disorder. This study utilized a secret shopper telephone survey to contact almost all outpatient mental health treatment facilities in the contiguous United States. We estimated multivariable regressions to examine county- and facility-level predictors of offering services for children with autism spectrum disorder. We found that 50.3% of 6156 outpatient facilities reported offering care for children with autism spectrum disorder. Non-metro counties, counties with a lower percentage of non-White residents, counties with a higher percentage of uninsured residents, and counties with a higher poverty rate had fewer outpatient mental health treatment facilities providing care for children with autism spectrum disorder. Facilities accepting Medicaid as a form of payment, offering telehealth, and private for-profit facilities were more likely to provide services for children with autism spectrum disorder. Only half of outpatient mental health treatment facilities offered care for children with autism spectrum disorder, and both rural and lower socioeconomic status counties were less likely to have a facility offering care for children with autism spectrum disorder.