This regiment was organized at Camp Terry, Mattoon, Coles County, Illinois, by Colonel James Monroe, who at the time was major of the 7th Illinois Infantry. Companies A, C, D, H, I and K were from Coles County; B from Cumberland; E from Clark; F and G from Clark and Crawford. As a colonel in 1861, Ulysses S. Grant organized his first command, the 21st Illinois Volunteer Infantry Regiment, in Mattoon.
It was mustered into service on September 6, 1862, with James Monroe as Colonel, Jonathan Biggs, of Westfield, Clark County, as Lieutenant Colonel, and James A. Connolly, of Charleston, Illinois, as Major. On the 19th of September 1862, the Regiment was loaded into freight cars at Mattoon, and transported to Louisville, Kentucky, where it was at once put to work, under Major General William "Bull" Nelson, to fortify the city against Confederate General Braxton Bragg, who was then advancing on it in pursuit of Union General Don Carlos Buell.
On October 1, having been assigned to the Thirty-third Brigade (General William R. Terrill), Fourth Division (General James S. Jackson), in McCook's Corps, the regiment started on the march under Buell, southward through Kentucky, after Bragg, who had turned back, and up to this time the regiment never had battalion drill, and hardly an attempt at company drill, as all the officers, except the colonel, were "raw recruits".
Just 19 days after leaving Mattoon, the regiment engaged in the Battle of Perryville, where 36 men were killed in action and 180 wounded. Generals Terrill and Jackson were both killed immediately behind and within twenty feet of the line of the regiment. Among the wounded were Captain Coblentz of Company E, First Lieutenant S. M. Shepard of Company A and Adjutant Leander H. Hamlin. Following the devastating bloodshed the 123rd was assigned to protect the railroad bridge across the Green River at Munfordville, Kentucky, in November and December 1862. It has become known as the "Battle for the Bridge," which began when the Union garrison led by John T. Wilder, prior to his assuming command of the 123rd, surrendered during the Battle of Munfordville.
The 123rd was initially assigned to the Army of the Ohio from September 1862 to November 1862, and then to the Army of the Cumberland from November 1862 to June 1865. The commanding general of the Army of the Cumberland was William Rosecrans. The regiment was in the 1st Brigade with Colonel Albert S. Hall, which was part of the Fifth Division commanded by Brigadier General Joseph J. Reynolds, reporting to Major General George Henry Thomas. Later, for a time, the 123rd Illinois Infantry Regiment was attached to the mounted unit known as the "Wilder Lightning Brigade" commanded by Wilder. It was assigned to the 1st Brigade, 4th Division, 14th Army Corps.
The regiment was mounted from May 1863 to November 1864. Spencer carbines, invented by Christopher Spencer, were introduced at this time. In combat, the carbines, shorter and lighter than the rifle version, were first used in the Battle of Hoover's Gap.
During the Battle of Selma, First Lieutenant O. J. McManus, Sergeants J. S. Mullen and Henry E. Cross, Corporal McMurry and Privates Daniel Cook, John Bowman, Marion White and Henry Woodruff were killed, with 50 wounded, including Lieutenant Colonel Biggs, Adjutant L. B. Bane, Captains W. E. Adams and Owen Wiley, Lieutenants Alex. McNutt and J. R. Harding.
Late in the war, the regiment pursued Confederate General John Bell Hood. The unit was instrumental in the capture of former Confederate capital Montgomery, Alabama.
In June 1865 new recruits and some veterans were transferred to the 61st Regiment as the 123rd prepared to disband. Those who remained were mustered out June 27, 1865, by Captain L. M. Hosea and formally discharged at Springfield, Illinois, on July 11, 1865.
The regiment lost during service three officers and 82 enlisted men killed and mortally wounded and one officer and 133 enlisted men by disease for a total of 219.
Colonel James Monroe: September 6, 1862 - October 7, 1863 (killed at the Battle of Farmington)Surgeon John Milton Phipps, July 2, 1863 - March 30, 1864.
Sergeant John Hamilton Morgan
Brigadier General of Volunteers James S. Jackson: October 1, 1862 - October 8, 1862 (killed at the Battle of Perryville)
Brigadier General of Volunteers William R. Terrill: September 9, 1862 - October 8, 1862 (killed at the Battle of Perryville)
Lt. Colonel James A. Connolly
Captain Oscar R. Bane
Colonel John T. Wilder: May 6, 1863 - November 1864 (resignation)
Brigadier General Kenner Garrard: May 20, 1864 - October 28, 1864
Major General James H. Wilson: October 28, 1864 - June 27, 1865