Mental Health Transparency Act This bill requires private health insurance plans to publish information about the number and percentage of behavioral health care and substance use disorder treatment providers located in the service area of the plan that are in-network. Additionally, the Department of Health and Human Services (HHS) must establish designations to reflect the breadth of in-network coverage for each type of provider in a service area. HHS also must publish specified information about each behavioral health care and substance use disorder treatment provider in the country, including the location of each provider and the services each provider furnishes.