Mandatory sentencing requires that offenders serve a predefined term for certain crimes, commonly serious and violent offenses. Judges are bound by law; these sentences are produced through the legislature, not the judicial system. They are instituted to expedite the sentencing process and limit the possibility of irregularity of outcomes due to judicial discretion. Mandatory sentences are typically given to people who are convicted of certain serious and/or violent crimes, and require a prison sentence. Mandatory sentencing laws vary across nations; they are more prevalent in common law jurisdictions because civil law jurisdictions usually prescribe minimum and maximum sentences for every type of crime in explicit laws.

Mandatory sentencing laws often target "moral vices" (such as alcohol, sex, drugs) and crimes that threaten a person's livelihood. The idea is that there are some crimes that are so heinous, there is no way to accept the offender back into the general population without first punishing them sufficiently. Some crimes are viewed as serious enough to require an indefinite removal from society by a life sentence, or sometimes capital punishment. It is viewed as a public service to separate these people from the general population, as it is assumed that the nature of the crime or the frequency of violation supersedes the subjective opinion of a judge. Remedying the irregularities in sentencing that arise from judicial discretion is supposed to make sentencing more fair and balanced. In Australia and the United Kingdom, sentencing has been heavily influenced by judicial idiosyncrasies. Individual judges have a significant effect on the outcome of the case, sometimes leading the public to believe that a sentence reflects more about the judge than the offender. Subsequently, creating stricter sentencing guidelines would promote consistency and fairness in the judicial system. Mandatory sentences are also supposed to serve as a general deterrence for potential criminals and repeat offenders, who are expected to avoid crime because they can be certain of their sentence if they are caught. This is the reasoning behind the "tough on crime" policy.

United States federal juries are generally not allowed to be informed of the mandatory minimum penalties that may apply if the accused is convicted because the jury's role is limited to a determination of guilt or innocence. However, defense attorneys sometimes have found ways to impart this information to juries; for instance, it is occasionally possible, on cross-examination of an informant who faced similar charges, to ask how much time he was facing. It is sometimes deemed permissible because it is a means of impeaching the witness. However, in at least one state court case in Idaho, it was deemed impermissible.

Notably, capital punishment has been mandatory for murder in a certain number of jurisdictions, including the United Kingdom until 1957 and Canada until 1961.