Two of the classic prerogative writs serve very different functions, even though both are often filed at the appellate level.
Mandamus and certiorari are frequently mentioned in the same breath because both trace back to the same family of English common-law prerogative writs, but they serve fundamentally different purposes.
Mandamus: compelling action
Mandamus commands a public official, agency, or lower court to perform a clear legal duty it has failed to perform. It is forward-looking — it makes something happen that has not yet happened.
Certiorari: reviewing a completed decision
Certiorari, by contrast, is a mechanism for reviewing a decision that a lower court or tribunal has already made, typically to determine whether that body acted within its lawful authority and followed proper procedure. It is backward-looking — it examines something that has already occurred.
Certiorari at the U.S. Supreme Court
The most familiar modern use of certiorari is the U.S. Supreme Court's discretionary review process: a "petition for a writ of certiorari" asks the Court to review a lower court's judgment, and the Court grants only a small fraction of the petitions it receives each year.
Overlapping use in some state systems
Some states, like New York, have folded the certiorari function into a broader unified proceeding (Article 78) rather than maintaining it as a separately named writ, which can create confusion for anyone comparing terminology across jurisdictions.
Choosing the right vehicle
If the problem is that an official has not yet acted, mandamus is the relevant tool. If the problem is that a tribunal already reached a decision and the petitioner believes that decision exceeded its authority or violated proper procedure, certiorari (or its modern statutory equivalent) is the more appropriate vehicle.
Filings, deadlines, and what a grant actually produces
The functional difference shows up in the paperwork. A mandamus petition is an original proceeding filed against the judge or officer who failed to act; a certiorari petition asks a higher court to take up a decision already made and review it on the record.
| Mandamus in a court of appeals | Certiorari in the Supreme Court | |
|---|---|---|
| Authority | 28 U.S.C. § 1651; Fed. R. App. P. 21 | 28 U.S.C. § 1254(1) from a court of appeals; § 1257(a) from a state court of last resort |
| What must be shown | A clear and indisputable right, no other adequate means to obtain relief, and that the writ is appropriate in the circumstances | Compelling reasons under Sup. Ct. R. 10: a conflict among courts of appeals or with a state court of last resort, or an important unsettled question of federal law |
| When it is filed | While the case is still before the lower court; the rule sets no deadline, but delay is itself a reason to deny | Within 90 days of entry of the judgment, Sup. Ct. R. 13.1; a Justice may extend the time under Sup. Ct. R. 13.5 |
| Who responds | Nobody, unless ordered: under Fed. R. App. P. 21(b)(1) the court may deny without an answer, and it may not grant relief until an answer has been ordered | The respondent may file a brief in opposition; the case is distributed for conference |
| What a grant produces | An order directing the lower court or officer to act, or to vacate what it did | Plenary review, ending in affirmance, reversal, or remand |
| What a denial decides | Often nothing: denials are frequently unexplained and leave the issue for appeal | Nothing on the merits; it is not an endorsement of the decision below |
The Supreme Court also entertains original writs, but Sup. Ct. R. 20.1 states that they are rarely granted and require exceptional circumstances plus the absence of adequate relief in any other form or from any other court.
When review on the record is the right vehicle and mandamus is not
Certiorari, in its older sense, brings up the record of a completed adjudication. That is the correct vehicle whenever the grievance is the quality of a decision rather than the absence of one, and several states preserve the function under other names.
- New York. C.P.L.R. Article 78 merged mandamus, prohibition, and certiorari into one proceeding, but § 7803 keeps the questions separate: whether a determination was arbitrary and capricious or affected by an error of law, and, where a hearing was held, whether the determination is supported by substantial evidence.
- California. Cal. Civ. Proc. Code § 1094.5 provides administrative mandate for decisions made after a required evidentiary hearing, which is record review; § 1085 remains for the duty that was never performed.
- Federal agencies. The equivalent of certiorari is review of final agency action under 5 U.S.C. § 706(2), decided on the administrative record.
The practical rule: if a tribunal has ruled and the complaint is that it ruled badly, the answer is record review or appeal. Mandamus is for the ruling that never came.
The limits shared by both petitions
Neither petition creates a right to be heard. Sup. Ct. R. 10 states plainly that review on certiorari is not a matter of right but of judicial discretion, and mandamus carries the same reservation: a court may deny it although every element is met. Neither writ reweighs evidence, and neither substitutes for a step the petitioner skipped.
Two consequences follow for anyone choosing between them. A mandamus petition does not preserve a missed certiorari deadline, because the writ is not a device for reviving lapsed review. And an unexplained denial of either petition settles nothing about the underlying question, so the same argument can return in the ordinary appeal without any preclusive effect from the earlier refusal.
Key points
- Mandamus is an original proceeding against the officer or judge; certiorari asks a higher court to review a completed decision on the record.
- A certiorari petition is due within 90 days under Sup. Ct. R. 13.1, while Fed. R. App. P. 21 sets no deadline for a mandamus petition.
- Under Fed. R. App. P. 21(b)(1) a court may deny a petition without an answer but may not grant relief until it has ordered one.
- Sup. Ct. R. 10 makes review discretionary and lists conflict among courts and important unsettled federal questions as the compelling reasons.
- When a tribunal has already decided after a hearing, the vehicle is record review — N.Y. C.P.L.R. § 7803, Cal. Civ. Proc. Code § 1094.5, or 5 U.S.C. § 706(2) — not mandamus.