To identify the state-level policies and policy domains that state policymakers and advocates perceive as most important for positively impacting the use of children's mental health services (CMHS). We used a modified Delphi technique (i.e., two rounds of questionnaires and an interview) during Spring 2021 to elicit perceptions among state mental health agency officials and advocates (n = 28) from twelve states on state policies that impact the use of CMHS. Participants rated a list of pre-specified policies on a 7-point Likert scale (1 = not important, 7 = extremely important) in the following policy domains: insurance coverage and limits, mental health services, school and social. Participants added nine policies to the initial list of 24 policies. The "school" policy domain was perceived as the most important, while the "social" policy domain was perceived as the least important after the first questionnaire and the second most important policy domain after the second questionnaire. The individual policies perceived as most important were school-based mental health services, state mental health parity, and Medicaid reimbursement rates. Key stakeholders in CMHS should leverage this group of policies to understand the current policy landscape in their state and to identify gaps in policy domains and potential policy opportunities to create a more comprehensive system to address children's mental health from a holistic, evidence-based policymaking perspective.