From 1941 to 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. As Governor, McGrath presided over a limited-purpose state constitutional convention in 1944. McGrath was elected as a Democrat to the United States Senate from Rhode Island in 1946 to join a Congress, where the opposition Republican Party had just replaced Democratic majorities in both houses. He was briefly chairman of the U.S. Senate Committee on the District of Columbia for the 81st Congress. 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.
Chairman of Democratic National Committee
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.
Truman appointed McGrath Attorney General of the United States on August 24, 1949. After McGrath had refused to co-operate in a corruption investigation initiated by his own department. Truman asked for and received McGrath's resignation on April 3, 1952. Alternative accounts have contradictorily suggested that after a meeting of the Joint Chiefs of Staff at Truman's "Little White House" in Key West, the Secretary of the Navy, along with other members, had threatened to resign if they, too, were forced to comply with Special Assistant Attorney General Newbold Morris's request for the personal records of all members who might have received gifts under the scope of the corruption investigation. Under pressure to follow through with the Justice Department corruption investigation, along with the threats of resignation, McGrath agreed that Morris's request was asking too much and that the best thing to do was to clean up the department from that point forward and leave the past alone. Truman had been backed into a corner, and the only way out was to ask for McGrath's resignation. That account was corroborated by a letter from Truman to McGrath, which hung in the hallway of McGrath's summer home in Narragansett, Rhode Island up to the time of his death in 1966. 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, losing the Democratic primary to Claiborne Pell. McGrath died of a heart attack in Narragansett, Rhode Island on September 2, 1966, at the age of 62. His body was buried at the in Pawtucket, Rhode Island. There is a bust of Senator McGrath outside the House chamber in the Rhode Island State House.
