Writing for the Court, Justice John Harlan I held that the defendant, Thompson, was entitled to a trial by a jury of twelve persons under the Sixth Amendment of the United States Constitution. Harlan also argued that Utah’s state law that allowed for smaller juries in certain cases could not be applied to Thompson’s crime, which was committed when Utah was still a territory because it would be an unconstitutional ex post facto application of the law. He wrote, “[i]f, in respect to felonies committed in Utah while it was a Territory, it was competent for the State to prescribe a jury of eight persons, it could just as well have prescribed a jury of four or two, and, perhaps, have dispensed altogether with a jury, and provided for a trial before a single judge.”

Justice Harlan emphasized that the right to a trial by jury, as understood at the time of the adoption of the Constitution, meant a jury composed of twelve persons. This interpretation was based on historical practices dating back to England and the early American colonies, where a jury traditionally consisted of twelve members. Harlan argued that the Sixth Amendment's guarantee of a jury trial was meant to preserve this traditional jury size, which was fundamental to the Anglo-American system of justice. He explained, “[b]ut the wise men who framed the Constitution of the United States and the people who approved it were of opinion that life and liberty, when involved in criminal prosecutions, would not be adequately secured except through the unanimous verdict of twelve jurors. It was not for the State, in respect of a crime committed within its limits while it was a Territory, to dispense with that guarantee simply because its people had reached the conclusion that the truth could be as well ascertained, and the liberty of the accused be as well guarded, by eight as by twelve jurors in a criminal case.” The Court reversed Thompson's conviction, ruling that the use of an eight-person jury violated his constitutional rights and remanding the case for further proceedings in line with the Court’s opinion.