A noted engineer, he constructed a series of forts to protect the Bozeman Trail, but suffered a major defeat at the hands of the warchief Red Cloud.

In August 1862, amid a pressing need for troops, Indiana Governor Oliver P. Morton asked Secretary of War Edwin M. Stanton to send an experienced military bureaucrat to organize new volunteer regiments.

Carrington continued to investigate secret organizations (Knights of the Golden Circle) that harbored deserters, discouraged enlistments, and obstructed the draft.

His efforts as a spymaster were recognized by the new commander of the Northern Department, Major General Samuel P. Heintzelman, who relied on him and his intelligence findings.

Whereas Carrington personally believed that leaders of the secret groups should be arrested and tried in federal civil courts for obstructing the war effort, his wishes were overruled by Governor Morton, Secretary Stanton, and, tacitly, President Lincoln, who chose to try conspirators in military commissions.

Assigned to protect the Bozeman Trail, he built and personally manned the remote Fort Phil Kearny during Red Cloud's War.

In 1868, Margaret Carrington (his wife) published her story about Fort Phil Kearny in a book titled Absaraka, home of the Crows.

[4] In 1870, Carrington retired from active service and was appointed professor of military science at Wabash College in Indiana, serving until 1878 when he moved to Hyde Park in Boston, Massachusetts.

[6] The actor Roy Engel played Carrington in the 1958 episode, "Old Gabe," referring to a nickname of the famous mountain man Jim Bridger, on the syndicated anthology series, Death Valley Days, hosted by Stanley Andrews.

Harry Shannon played an aging Jim Bridger, who returns home to Missouri to find that his wife has died in childbirth, and son Felix (Ron Hagerthy) is trying to keep their farm despite an unpaid mortgage.