He graduated from the United States Military Academy in 1880 with the rank of second lieutenant and was assigned to the 4th Cavalry at Fort Hayes, where he served in the American Indian Wars.

He then served in the Pancho Villa Expedition from 1915 to 1916 and led the 6th Infantry Division during World War I.

[1] He was awarded the Army Distinguished Service Medal, the citation for which reads: The President of the United States of America, authorized by Act of Congress, July 9, 1918, takes pride in presenting the Army Distinguished Service Medal (Posthumously) to Brigadier General James Brailsford Erwin, United States Army, for exceptionally meritorious and distinguished services to the Government of the United States, in a duty of great responsibility during World War I.

With sound technical skill, initiative, and untiring energy, General Erwin assisted in the organization and training of the 6th Division, and commanded with distinction the 12th Infantry Brigade during its operations in the Vosges sector, and during the Meuse-Argonne offensive, 1 to 11 November 1918.

His rare quality of leadership and unremitting devotion to duty were material factors in the successful operations of his division, contributing markedly to the accomplishments of the American Expeditionary Forces in Europe.