Born on December 19, 1882, in Boherbue, Cork, Ireland, Mahoney received an Artium Baccalaureus degree in 1905 from Brown University and a Bachelor of Laws in 1908 from Harvard Law School.

He served as an Assistant United States Attorney for the District of Rhode Island from 1917 to 1921.

[2] Mahoney was nominated by President Franklin D. Roosevelt on May 24, 1935, to a seat on the United States District Court for the District of Rhode Island vacated by Judge Ira Lloyd Letts.

[1] Mahoney was nominated by President Roosevelt on January 11, 1940, to a seat on the United States Court of Appeals for the First Circuit vacated by Judge James Madison Morton Jr.

He assumed senior status due to a certified disability on December 20, 1950.