Mens Rea Reform Act of 2018 This bill establishes the National Criminal Justice Commission to conduct a comprehensive review of the criminal justice system; make recommendations for criminal justice reform; and disseminate findings and guidance to the federal government and to state, local, and tribal governments. Additionally, the bill amends the federal criminal code to establish a default mens rea (i.e., guilty mind) requirement for federal criminal statutory and regulatory offenses that lack a state of mind requirement. For a new criminal statutory or regulatory offense, the government must prove that a defendant acted willfully with respect to each element of the offense that lacks a specific state of mind requirement. For an existing criminal statutory or regulatory offense, the bill establishes requirements for the Department of Justice, executive agencies, and the National Criminal Justice Commission with respect to determining the applicable state of mind requirement.