Health coverage of early intensive behavioral intervention (EIBI) for children with autism spectrum disorder (ASD) is rapidly expanding across the United States. Yet we know little about the time-lag between diagnosis and treatment onset. We integrated administrative, Medicaid claims, and Census data for children in an EIBI Medicaid waiver (n = 473) to examine the relationship between time-lag and (a) child race-ethnicity and (b) neighborhood racial composition, poverty, affluence, and urbanicity. We explored whether the relationship between child race-ethnicity and time-lag varies by neighborhood characteristics. Average time-lag between diagnosis and treatment onset was nearly 3 years. Child race-ethnicity and neighborhood characteristics did not predict time-lag. Reducing time-lag is critical to ensuring that children with ASD receive treatment as early as possible.