Tuesday's unanimous decision limits one avenue that lawyers, including those representing prisoners who have been sentenced to death, have used that can delay the appeals process.
The high court overturned rulings by two federal appeals courts in which appeals by two death row inmates were halted because their mental conditions had left them unable to assist their lawyers with their defenses.
Writing for the court, Justice Clarence Thomas said the lower courts erred in relying on two federal statutes to find a requirement for competency, which he said does not flow from a defendant's right to counsel under the Sixth Amendment to the U.S. Constitution.
Thomas said a district judge who believes an incompetent defendant could substantially aid in his defense should examine the likelihood that the defendant will regain competence.
But he said lawyers are "quite capable" without their clients' help to review cases, and identify possible arguments or state court errors that can be raised on appeal.
"Where there is no reasonable hope of competence, a stay is inappropriate and merely frustrates the state's attempts to defend its presumptively valid judgment," Thomas wrote.
The court also said it was "unwarranted" to extrapolate a definitive rule based on a 1960s case involving an incompetent death row inmate that it put on hold for nearly three decades. That case ended when the prisoner died.
One of the cases decided on Tuesday involved Ernest Valencia Gonzales, who was convicted by an Arizona jury in the stabbings of two people, one of whom died, in front of their seven-year-old son during a burglary at their home.
Dale Baich, who works in the federal public defender's office that represented Gonzales, said the Supreme Court decision left room for federal courts to put some appeals on hold. A prisoner's competency to assist counsel is an issue in roughly one dozen capital cases pending nationwide, he added.
The other involved Sean Carter, who an Ohio jury convicted over the rape and stabbing death of his adoptive grandmother. A lawyer for Carter was not immediately available for comment.
The cases are Ryan v. Gonzales, U.S. Supreme Court, No. 10-930; and Tibbals v. Carter, U.S. Supreme Court, No. 11-218.
(Reporting by Jonathan Stempel; Editing by Jackie Frank)