McCarthy was born in Grand Chute, Wisconsin, on November 14, 1908, and educated at Marquette University. He practiced law in Wisconsin until 1939, when he was elected circuit-court judge. During World War II he served in the U.S. Marine Corps, attaining the rank of captain during service in the Pacific theater of operations. In 1946 he was elected on the Republican ticket to the U.S. Senate and was reelected in 1952.

McCarthy first attracted national attention in February 1950, with the charge that the Department of State had been infiltrated by Communists. Although his accusation was never substantiated, during the next three years he repeatedly accused various high-ranking officials, intellectuals and the Hollywood establishment of subversive activities. McCarthy pursued alleged communists with a fervor. He was a master at controlling the media and received enormous publicity. Suspected communists would be hauled in front of McCarthy's committee like a common criminals, often with no evidence at all. McCarthy's chief prosecutor was a New York Lawyer named Roy Cohn. Cohn has obnoxious and abrasive. He hated everyone, especially communists and homosexuals. When Cohn and McCarthy had someone in front of their committee they were merciless. They demanded that the "witness" turn in other suspected communists. Since most of the accused were never involved in any communist activity this was difficult. McCarthy would scream "I have a list" and wave a piece of paper demanding information. The list was never made public. Thousands of Americans lost jobs and careers during McCarthy's witch hunts.

In 1953, as chairman of the Senate subcommittee on investigations, McCarthy continued his probe of alleged Communist activities, and in April 1954 he accused the secretary of the army of concealing foreign espionage activities. In rebuttal the secretary stated that members of the subcommittee staff had threatened army officials in efforts to obtain preferential treatment for a former unpaid consultant of the subcommittee who had been drafted. The secretary of the army had tapes of telephone conversations that recorded Cohn and McCarthy harassing army officials and threatening them with investigations. The tapes also revealed that Cohn had demanded that his assistant G. David Schine, not be called to military service. When Schine was called anyway McCarthy then made his accusations against the army. When the Secretary of the army was called to testify McCarthy attacked a lawyer that had worked for the secretary's chief counsel (lawyer). The army lawyer Mr. Welch, declared "Until this moment, Senator, I think I never gauged your cruelty or your recklessness...Have you no sense of decency sir? At long last, have you no sense of decency?" McCarthy was exposed for the fraud he was.

During the ensuing Senate investigations, which were widely publicized in the press and given nationwide radio and television coverage, McCarthy was cleared of the charges against him but was censured (punished) by the Senate for the methods he had used in his investigations and for his abuse of certain senators and Senate committees. His influence both in the Senate and on the national political scene diminished steadily thereafter. McCarthy remained in the Senate until his death in Bethesda, Maryland, on May 2, 1957. Roy Cohn outlived his mentor and returned to practice law in New York City. He specialized in getting big divorce settlements for rich women. Cohn lost his fortune when the IRS discovered he hadn't paid his taxes. Cohn died of AIDS on August 2, 1986.