Walsh was born in Leominster, Worcester County, Massachusetts. He attended the public schools there; graduated from Holy Cross College, Worcester, Mass., in 1893 and from Boston University Law School in 1897; was admitted to the bar and commenced practice at Fitchburg, Mass., in 1897, later practicing in Boston. He was a member of the State house of representatives, 1900-1901; lieutenant governor of Massachusetts 1913 and Governor 1914-1915; and a delegate at large to the Massachusetts constitutional convention in 1917 and 1918. He was elected as a Democrat to the United States Senate and served from March 4, 1919, to March 3, 1925; unsuccessful candidate for reelection in 1924; resumed the practice of law in Boston; elected to the United States Senate on November 2, 1926, to fill the vacancy caused by the death of Henry Cabot Lodge and took his seat December 6, 1926; reelected in 1928, 1934 and 1940 for the term ending January 3, 1947; he was an unsuccessful candidate for reelection in 1946. During his Senate service, he held the posts of chairman of the Committee on Education and Labor (Seventy-third and Seventy-fourth Congresses), and the Committee on Naval Affairs (Seventy-fourth through Seventy-seventh and Seventy-ninth Congresses). He retired from political activities and resided in Clinton, Mass., until his death in Boston on June 11, 1947. He is buried in St. John’s Cemetery, Clinton, Mass.

Walsh was the first Irish-Catholic Governor of Massachusetts and also its first Irish-Catholic Senator. As Governor, he fought hard for a Women's suffrage Amendment to the Massachusetts constitution, but this effort failed. He also led the way toward establishing stricter film censorship in Massachusetts after large protests against D. W. Griffith's film The Birth of a Nation. In the Senate, he was an isolationist, opposing an American alliance with Great Britain up to the Attack on Pearl Harbor.

This article incorporates facts obtained from the public domain Biographical Directory of the United States Congress.

Preceded by: Eugene N. Foss Governor of Massachusetts 1914-1916 Succeeded by: Samuel W. McCall Preceded by: John W. Weeks U.S. Senator (Class 2) from Massachusetts 1919-1925 Served alongside: Henry Cabot Lodge, William M. Butler Succeeded by: Frederick H. Gillett Preceded by: William M. Butler U.S. Senator (Class 1) from Massachusetts 1926-1947 Served alongside: Frederick H. Gillett, Marcus A. Coolidge, Henry Cabot Lodge, Jr., Sinclair Weeks, Leverett Saltonstall Succeeded by: Henry Cabot Lodge, Jr. Governors of Massachusetts  Colony  Endicott • Winthrop • T. Dudley • Haynes • Vane • Winthrop • T. Dudley • Bellingham • Winthrop • Endicott • T. Dudley • Winthrop • Endicott • T. Dudley • Endicott • Bellingham • Endicott • Bellingham • Leverett • Bradstreet

 Dominion  J. Dudley • Andros • Bradstreet