associate justice of the Supreme Court of the United States (1930–45).

Roberts was the son of hardware merchant Josephus R. Roberts and Emma Lafferty Roberts. He graduated Phi Beta Kappa in 1895 from the University of Pennsylvania and then entered the university's law school, where he served as associate editor of the American Law Register (now University of Pennsylvania Law Record) and graduated with highest honours in 1898. Upon his graduation, he continued his association with the University of Pennsylvania for the next two decades, teaching contracts and property law, and he also engaged in private legal practice.

Roberts served briefly as assistant district attorney (1903–06) for Philadelphia county before returning to private legal practice. In 1918 he was appointed special deputy U.S. attorney to prosecute violations of the Espionage Act of 1917. Excelling in this position, Roberts drew the attention of Pres. Calvin Coolidge (Coolidge, Calvin), who in 1924 named him one of the two attorneys to prosecute parties named in the Teapot Dome Scandal that tarnished the administration of Pres. Warren G. Harding (Harding, Warren G.). After a methodical investigation, former Interior Secretary Albert Bacon Fall (Fall, Albert Bacon) was convicted of taking bribes in 1929. The following year, Pres. Herbert Hoover (Hoover, Herbert) had the opportunity to fill a pair of Supreme Court vacancies caused by the unexpected deaths of Chief Justice William Howard Taft (Taft, William Howard) and Justice Edward T. Sanford (Sanford, Edward T). While Charles Evans Hughes (Hughes, Charles Evans) won confirmation to the position of chief justice, Hoover's appointment of John J. Parker met with stiff opposition and was rejected by the Senate 41–39. Hoover subsequently nominated Roberts, who won unanimous confirmation from the Senate on May 20, 1930.

By the time Roberts joined the Supreme Court, the conservative majority that had been dominant in the 1920s had diminished, and the institution was clearly divided along ideological lines. With four reliable conservatives (George Sutherland (Sutherland, George), Pierce Butler (Butler, Pierce), James McReynolds (McReynolds, James), and Willis Van Devanter (Van Devanter, Willis)) holding only a slight numerical advantage over the more liberal bloc (Louis Brandeis (Brandeis, Louis), Oliver Wendell Holmes, Jr. (Holmes, Oliver Wendell, Jr.), and Harlan Fiske Stone (Stone, Harlan Fiske)), Hughes and Roberts entered the court as potential swing votes. As their service on the bench would reveal, Hughes and Roberts did often vote in similar fashion, leading some to regard their seemingly inevitable coupling—and ability to tilt decisions in a liberal or conservative direction—as the “Hughberts” vote. Often characterized as “unreliable members of the so-called liberal majority,” Roberts and Hughes cast the deciding vote on several New Deal programs of the Franklin D. Roosevelt (Roosevelt, Franklin D.) administration, upholding some and striking down others.

Roberts, a social liberal, made some of his most important contributions to the court in the area of civil liberties. A supporter of the doctrine of selective incorporation, Roberts voted to extend the authority of the Fourteenth Amendment, through its due process clause, to the states in order to protect individual rights from being infringed by both the federal and state governments. This tendency was most apparent in Stromberg v. California and Near v. Minnesota (both 1931), in which the court invalidated state-led attempts to restrict the First Amendment rights of speech and the press. In perhaps the most famous decision that he wrote, Herndon v. v. Lowry (1937), Roberts set aside the conviction of an African American communist organizer convicted under a law that provided no clear standard of guilt. In the area of economic and commerce law, Roberts's opinion in Nebbia v. New York (1934) upheld the price-setting activities of the New York State Milk Control Board and provided a legal foundation for government regulation of business “affected with a public interest.” This liberal orientation was also apparent in Roberts's decisions to uphold the National Labor Relations Act of 1935 (commonly known as the Wagner Act), the Social Security Act of 1935, and the Fair Labor Standards Act of 1938. However, in a series of cases involving prized elements of the New Deal, Roberts sided with the economic conservatives in declaring unconstitutional the Railroad Retirement Act, the National Industrial Recovery Act, the Agricultural Adjustment Act, and the Bituminous Coal Conservation Act.

Despite his role in the New Deal cases, however, Roberts is best remembered for his role as the famous “switch in time that saved nine.” In a move assumed to have been politically motivated, Chief Justice Hughes is widely believed to have persuaded Roberts to change his vote in the case of West Coast Hotel Co. v. Parrish (1937), in which the court upheld Washington state's minimum wage law. The ruling also signaled that the remainder of the New Deal legislation would be declared constitutional and helped to undermine the momentum behind Roosevelt's court-reorganization (“court-packing”) plan.

Roberts's tenure on the court also included a stint overseeing commissions that investigated the attack on Pearl Harbor in 1941 and the theft of art objects by the Germans during World War II. Roberts retired from the Supreme Court on July 31, 1945, after which he served as dean of the University of Pennsylvania Law School and as chair of the Security Board of the Atomic Energy Commission and the Fund for the Advancement of Education. His 1951 Oliver Wendell Holmes lectures for Harvard University were published that same year under the title The Court and the Constitution.