Pat McCarran was born in Reno, Nevada. After graduating from the University of Nevada, Reno, he worked in farming. In 1903 he became a member of the State legislature and after studying for a law degree he eventually became district attorney of Nye County (1907-09).

McCarran was also Nevada Chief Justice (1917-18), chairman of the Nevada State Board of Parole Commissioners (1913-18) and chairman of the Nevada State Board of Bar Examiners (1919-32). A member of the Democratic Party, McCarran, after two unsuccessful bids in 1916 and 1926, was elected the U.S. Senate in 1932.

He sponsored laws concerned with the nation's security, including the Civil Aeronautics Act of 1938; the Federal Airport Act of 1945; the Administrative Procedures Act of 1946. He was also an early advocate of a separate Air Force.

After the Second World War, McCarran established himself as the Senate's most powerful anti-Communist. He was the chairman of the Senate Internal Security Subcommittee that investigated the administrations headed by Franklin D. Roosevelt and Harry S. Truman. In September 1950 he was the chief sponsor of the McCarran-Woods Act. This legislation required registration with the Attorney General of the American Communist Party and affiliated organizations.

In 1950, McCarran assisted James A. Vaughn, a Nevada resident, In the Matter of Vivienne Wilson in Vaughn's attempt to nationalize his Swedish-Japanese wife and daughter who had been barred from immigrating due to racial obstacles. Private legislation was signed into law on 8 August 1950. Tragically, Vaughn's wife died in Japan on that day. (see also Sweden v. Yamaguchi) McCarran's assistance in this tragic matter was thought to shape his sympathies in opposition to racial quotas.

In June 1952, McCarran joined Francis Walter in instigating the passing of the McCarran-Walter Act that imposed more rigid restrictions on entry quotas to the United States. It also stiffened the existing law relating to the admission, exclusion and deportation of dangerous aliens defined by the Internal Security Act. In response to the act he made a well known statement:

"I believe that this nation is the last hope of Western civilization and if this oasis of the world shall be overrun, perverted, contaminated or destroyed, then the last flickering light of humanity will be extinguished. I take no issue with those who would praise the contributions which have been made to our society by people of many races, of varied creeds and colors. America is indeed a joining together of many streams which go to form a mighty river which we call the American way. However, we have in the United States today hard-core, indigestible blocs which have not become integrated into the American way of life, but which, on the contrary are its deadly enemies. Today, as never before, untold millions are storming our gates for admission and those gates are cracking under the strain. The solution of the problems of Europe and Asia will not come through a transplanting of those problems en masse to the United States.... I do not intend to become prophetic, but if the enemies of this legislation succeed in riddling it to pieces, or in amending it beyond recognition, they will have contributed more to promote this nation's downfall than any other group since we achieved our independence as a nation." (Senator Pat McCarran, Cong. Rec., March 2, 1953, p. 1518.) The act was later overturned by the 1965 Immigration Act. Pat McCarran remained in the Senate until his death in Hawthorne, Nevada in 1954.