How Policymakers Innovate Around Behavioral Health: Adoption of the New Mexico “No Behavioral Health Cost-sharing” Law

State policymakers have long sought to improve access to mental health and substance use disorder (MH/SUD) treatment through insurance market reforms. Examining decisions made by innovative policymakers (“policy entrepreneurs”) can inform the potential scope and limits of legislative reform. Beginning in 2022, New Mexico became the first state to eliminate cost-sharing for MH/SUD treatment in private insurance plans subject to state regulation. Based on key informant interviews (N = 30), this study recounts the law’s passage and intended impact. Key facilitators to the law’s passage included receptive leadership, legislative champions with medical and insurance backgrounds, the use of local research evidence, advocate testimony, support from health industry figures, the severity of mental health and substance use disorders, and increased attention to MH/SUD during the COVID-19 pandemic. Findings have important implications for states considering similar laws to improve access to MH/SUD treatment.