McGrath, a Democrat, served as U.S. Attorney for Rhode Island before becoming Governor, U.S. Solicitor General, U.S. Senator, chairman of the Democratic National Committee and Attorney General of the United States.
McGrath was born in Woonsocket, Rhode Island. He graduated from La Salle Academy, Providence in 1922, from Providence College in 1926, and from Boston University in 1929. From 1930 to 1934 he was the city solicitor of Central Falls, Rhode Island. During this time he was also interested in the real estate, insurance, and banking industries. He served as United States Attorney for the District of Rhode Island from 1934 to 1940.
From 1940 until 1945, he was Governor of Rhode Island, reorganizing the juvenile court system while sponsoring a workers' compensation fund and a labor relations board, but he resigned in the middle of his third term to accept appointment as Solicitor General of the United States (1945-46).
McGrath was elected as a Democrat to the United States Senate from Rhode Island in 1946, joining an 80th Congress (1947-49) that had unaccustomed Republican majorities in both houses. He was briefly chairman of the U.S. Senate Committee on the District of Columbia for the 81st Congress (to which the 1948 election had returned Democratic majorities). In the Senate, McGrath opposed reducing wartime economic controls and taxes, wishing to spend the latter instead on Social Security, national health insurance and education.
He was chairman of the Democratic National Committee from 1947 to 1949. In managing President Harry Truman's successful 1948 election campaign, McGrath alienated white Southerners but won over crucial black constituencies by integrating the Democratic national headquarters staff.
President Truman appointed Sen. McGrath Attorney General of the United States on August 24, 1949. He resigned on April 3, 1952, after he had refused to cooperate in a corruption investigation initiated by his own department. Truman asked for and received McGrath's resignation.
McGrath entered the private practice of law in Washington, D.C., and Providence. In 1960, he was an unsuccessful candidate to succeed the retiring U.S. Sen. Theodore Francis Green (Democrat of Rhode Island), losing the Democratic primary (also contested by former Gov. Dennis J. Roberts) to Claiborne Pell.