Mandatory Law and Legal DefinitionMandatory refers to something that is required, and not optional or subject to discretion. In legal construction of statutes, mandatory requirements of law are typically found by the use of words such as "must", "will" and "shall". For example, if a statute provides that after a person files a petition, "a hearing shall be held", then a hearing on the petition must be held. Mandatory sentencing in criminal statutes has been a controversial topic due to prison overcrowding. Sentence reform advocates argue that judges should have discretion in determining sentences based upon multiple factors in individual cases. States have statutes, which vary by state, requiring mandatory reporting of child abuse and neglect.