He served as the tenth Governor of Wisconsin and represented the United States as Minister to Spain under presidents Rutherford B. Hayes and James A. Garfield.

In 1858, with the American Civil War looming, Fairchild volunteered as a private with the Wisconsin militia company known as the "Governor's Guard".

Electing to stay with the 2nd Wisconsin Infantry, Fairchild was soon commissioned lieutenant colonel and, with his regiment now a part of the famed Iron Brigade within the Army of the Potomac, participated in the Second Battle of Bull Run.

During the engagement, at approximately 10:00, the 2nd Wisconsin Infantry delivered a striking blow by capturing very first Confederate general officer of the war, Brig.

Almost immediately after this success, the regiment was ambushed by an attack on their right flank, losing seventy-seven percent of their ranks, including most officers.

While recovering from his amputated left arm, Fairchild was commissioned as a brigadier general by President Abraham Lincoln on October 19, 1863, but the appointment was not confirmed by the Senate.

[1][2] Fairchild later served as Commander-in-Chief of the Grand Army of the Republic (1886–1887) and of the Military Order of the Loyal Legion of the United States (1893–1895); during the former of these terms, he came to national prominence for calling upon God to "palsy" President Grover Cleveland for offering to return several captured Confederate battle flags to the veterans of their respective regiments as a peace gesture towards the South.

He supported Wisconsin Senator Timothy O. Howe in pushing for more severe measures in the South that would guarantee civil rights by force.