He served in the United States Army as a lieutenant in the Judge Advocate General's Department during World War I.

He was director of the State Department of Revenue and Regulation for Rhode Island in 1939.

He was a member and director of finance for the Rhode Island State Sinking Fund Commission from 1942 to 1946.

[1][2] Leahy was appointed on August 24, 1949, as a Democrat to the United States Senate to fill the vacancy caused by the resignation of United States Senator J. Howard McGrath and served from August 24, 1949, to December 18, 1950, a successor having been elected and qualified.

[1] Leahy was nominated by President Harry S. Truman on December 21, 1950, to a seat on the United States District Court for the District of Rhode Island vacated by Judge John Patrick Hartigan.