The Supreme Court ruled in favor of Gideon in a unanimous decision. Justice Black wrote the opinion for the Court, which ruled that the right to the assistance of counsel in felony criminal cases is a fundamental right, and thus must be required in state courts as well as federal courts. Justices Harlan and Clark wrote concurring opinions.

The Court rejected part of their prior decision in Betts v. Brady (1942). In that case, the justices had ruled that indigent defendants need only be provided with a lawyer under special circumstances. The decision accepted the portion of the Court’s ruling in Betts which stated that the parts of the Bill of Rights that are “fundamental and essential to a fair trial” are made binding on the states by the Due Process clause of the Fourteenth Amendment. They specifically noted, however, that “the Court in Betts was wrong … in concluding that the Sixth Amendment’s guarantee of counsel was not one of these fundamental rights.”

The Court said that the best proof that the right to counsel was fundamental and essential was that “[g]overnments … spend vast sums of money to … try defendants accused of crime … Similarly, there are few defendants charged with crime[s]… who fail to hire the best lawyers they can get to prepare and present their defenses.” This indicated that both the government and defendants considered the aid of a lawyer in criminal cases absolutely necessary.  In addition, the opinion noted that the Constitution places great emphasis on procedural safeguards designed to guarantee that defendants get fair trials. According to the opinion, “this noble idea cannot be realized if the poor man charged with a crime has to face his accusers without a lawyer to assist him.”  The Court concluded that the Sixth Amendment guarantee of a right to counsel was fundamental and essential to a fair trial in both state and federal criminal justice systems. In all felony criminal cases, states must provide lawyers for indigent defendants.

In his concurring opinion in Gideon, Justice Clark agreed that Betts v. Brady should be overturned, and that the Sixth Amendment must be interpreted to require states to provide counsel for criminal defendants. Under Betts, states were only required to provide lawyers for criminal defendants under special circumstances, which included capital cases. Justice Clark noted that the Constitution does not make any distinction between capital and noncapital cases, but requires procedural protections for defendants meeting the standard of due process of law in both situations. The procedural protections required therefore should not be different depending on whether the defendant was charged with a capital crime or a noncapital crime, according to Justice Clark.

In his concurring opinion, Justice Harlan also agreed that the right to counsel in criminal cases is a fundamental and essential right. He explained that Betts v. Brady mandated that there must be special circumstances present, such as complex charges, incompetence or illiteracy of defendants, or the possibility of the death penalty as a sentence, to require states to provide criminal defendants with counsel. He then argued that “the mere existence of a serious criminal charge constituted in itself special circumstances.” Since, according to Justice Harlan, all felony criminal trials involved special circumstances, states should be required to provide lawyers for indigent defendants.