On September 14, 1887—six months after hearing the oral arguments—the Supreme Court of Illinois filed its decision. Speaking for the six justices who heard the case, Benjamin Magruder, pictured here, stated "that neither in the record and proceedings aforesaid, nor in the rendition of the judgment aforesaid, is there anything erroneous, vicious, [or] defective," and he reaffirmed the lower court's decision. Magruder's assertion that the Supreme Court had "diligently examined and inspected" the record was convincing, for he went into remarkable detail in crafting his opinion, dealing with all the defendants and multiple pieces of evidence. The full opinion—some 219 pages in the original typescript—is available in the Haymarket Affair Digital Collection.

Magruder ruled that the key questions were whether the defendants had "a common purpose or design to advise, encourage, aid or abet the murder of the police," whether they acted to carry out this purpose or design, and whether they acted or spoke in such a way that had "the effect of encouraging, aiding, or abetting the crime in question." The answer to all of these questions was yes. In virtually every instance, Magruder interpreted the circumstantial evidence on which the conviction was based as incriminating proof, always giving the prosecution the benefit of the doubt. In Magruder's view, the bomb used in the Haymarket was clearly constructed by Louis Lingg. It was part of an unlawful conspiracy that included the International Working People's Association, the various anarchist subgroups in Chicago, and armed organizations like the Lehr- & Wehr-Verein.

In the opinion of the court, almost every detail of the Haymarket meeting—such as Spies's decision to locate it on Desplaines Street instead of the Haymarket itself, and the delay in starting—was not the result of accident or disorganization, but part of a plan. Magruder also said that the case against Neebe, considered by many to be questionable, was substantial enough to convict him. Since Neebe helped distribute some of the circulars advertising the meeting, Magruder added in a tricky use of the double negative, "we cannot say that the jury were not justified in holding him responsible" for the murder of Matthias Degan. Similarly, Judge Gary's instructions in the lower court were in order and the jury was competent.

One justice, John H. Mulkey, offered the only demurral. Stating that his health prevented him from filing a separate opinion, he said that he did not think the trial record was free from error. Given the number of defendants, however, the length of the proceedings, the amount of testimony, and the number of Gary's rulings, Mulkey marveled "that the errors were not more numerous and more serious than they are."

But Mulkey did not say what he thought the errors were, and in the end concurred in the opinion. He added that he was "fully satisfied that the conclusion reached vindicates the law, does complete justice between the prisoners and the State, and that it is fully warranted by the law and the evidence." This was perhaps an elegant way of saying that the ends justified the means, which probably reflected the feelings of many others both within and outside the legal profession.