He then entered private practice as an attorney, first in Saint Paul, Minnesota in 1914, and then in Boston, Massachusetts from 1915 until 1916.

Edgerton was a professor at Cornell Law School from 1916 until 1918, when he returned to private practice in Boston.

While at Cornell, Edgerton served as Special Assistant to the United States Attorney General from 1934 until 1935, during Franklin D. Roosevelt's first term as President.

[3] Edgerton was nominated by President Franklin D. Roosevelt on November 26, 1937, to an Associate Justice seat on the United States Court of Appeals for the District of Columbia (Judge of the United States Court of Appeals for the District of Columbia Circuit from June 25, 1948) vacated by Associate Justice Duncan Lawrence Groner.

[3] He served as Chief Judge and as a member of the Judicial Conference of the United States from May 28, 1955 to October 20, 1958.