After the death of Associate Justice Edward Terry Sanford in March 1930, President Herbert Hoover nominated John J. Parker to fill the vacancy on the court.

His decision to uphold the constitutionality of a state minimum wage law in West Coast Hotel Co. v. Parrish has been called "the switch in time that saved nine".

That term references the decision's possible role in the defeat of the Judicial Procedures Reform Bill of 1937, which would have expanded the Supreme Court and thus allowed Roosevelt to appoint Justices more sympathetic to his policies.

[a] He was one of three Justices, along with Robert H. Jackson and Frank Murphy, to vote against Roosevelt's orders for Japanese American internment camps in Korematsu v. United States as well as the lone judge to dissent in the case of Smith v. Allwright, which ruled white primaries unconstitutional.

Roberts was born in Philadelphia in 1875 and attended Germantown Academy and enrolled at the University of Pennsylvania at the age of 16 where he studied Greek, was elected to Phi Beta Kappa[2] and was the editor of The Daily Pennsylvanian.

[1] On the Court, Roberts was a swing vote between those, led by Justices Louis Brandeis, Benjamin Cardozo, and Harlan Fiske Stone, as well as Chief Justice Charles Evans Hughes, who would allow a broader interpretation of the Commerce Clause to allow Congress to pass New Deal legislation that would provide for a more active federal role in the national economy, and the Four Horsemen (Justices James Clark McReynolds, Pierce Butler, George Sutherland, and Willis Van Devanter) who favored a narrower interpretation of the Commerce Clause and believed that the Fourteenth Amendment Due Process Clause protected a strong "liberty of contract".

In 1936's United States v. Butler, Roberts sided with the Four Horsemen and wrote an opinion striking down the Agricultural Adjustment Act as beyond Congress's taxing and spending powers.

While Roberts is often accused of inconsistency in his jurisprudential stance towards the New Deal, legal scholars note that he had previously argued for a broad interpretation of government power in the 1934 case of Nebbia v. New York, and so his later vote in Parrish was not a complete reversal.

[10][11] In one of his notes from 1936, Hughes wrote that Roosevelt's re-election forced the court to depart from "its fortress in public opinion" and severely weakened its capability to base its rulings on personal or political beliefs.

[10] Roberts wrote the majority opinion in the landmark case of New Negro Alliance v. Sanitary Grocery Co., 303 U.S. 552 (1938), which safeguarded the right to boycott in the context of the struggle by African Americans against discriminatory hiring practices.

Perhaps influenced by his work on the Pearl Harbor commission, Roberts dissented from the Court's decision upholding internment of Japanese-Americans along the West Coast in 1944's Korematsu v. United States.

The second Roberts Commission was established in 1943 to consolidate earlier efforts on a national basis with the US Army to help protect monuments, fine arts, and archives in war zones.

He coined in that dissent the oft-quoted phrase that the frequent overruling of decisions "tends to bring adjudications of this tribunal into the same class as a restricted railroad ticket, good for this day and train only".

Roberts did prepare a short memorandum discussing his alleged change of stance around the time of the court-packing effort, which he left in the hands of Justice Felix Frankfurter.