Reed was born in Mason County, Kentucky. Upon receiving a B.A degree from both Kentucky Wesleyan University in 1902 and from Yale University in 1906, he studied law at the University of Virginia and Columbia University, and later studied in France.
After working as a lawyer in Maysville, Kentucky, he became general counsel of the Federal Farm Board from 1929 to 1932 and of the Reconstruction Finance Corporation from 1932 to 35. As the Solicitor General from 1935 to 1938, he presented the government arguments for numerous New Deal cases before the Supreme Court, where he was appointed to by President Franklin Delano Roosevelt. On the bench, Reed was generally considered a moderate and often held the balance between the liberal and the conservative members of the court in split decisions.
At the time of Reed's death in 1980 he was the longest lived Supreme Court Justice in American history.