The 99th Ohio Volunteer Infantry Regiment was organized at Camp Lima in Allen County, Ohio. It was mustered in for three-years service on August 26, 1862. Under the command of Colonel Albert Longworthy, the regiment was comprised of volunteers recruited in Allen, Auglaize, Hancock, Mercer, Putnam, Shelby, and Van Wert Counties.
From Camp Lima, the regiment was ordered to Lexington, Kentucky on August 31 to aid in the defense of Louisville. They were ordered to move to Cynthiana, Kentucky on September 3, and on to Covington, Kentucky and eventually to Louisville, Kentucky on September 17, all the while attached to the 23rd Brigade, 5th Division of the Army of the Ohio.
The regiment was attached to the 23rd Brigade, 5th Division, II Corps of the Army of the Ohio, placed under the command of Colonel Peter T. Swaine, and sent in pursuit of General Braxton Bragg, an operation that began on October 1. On October 8th, they were kept in reserve at the Battle of Perryville, and then continued to pursue Bragg’s troops through October 15th. On October 16, they marched to Nashville Tennessee, arriving there on November 7.
In Nashville, they were attached to the 3rd Brigade, 3rd Division, Left Wing, XIV Corps, of the Army of the Cumberland. They remained on duty in Nashville through December 26, when they were ordered to Murfreesboro, arriving there on December 30. Upon their arrival, the regiment fought in the Battle of Stones River December 30 through January 3, 1863.
Following the battle, the regiment was assigned to the 3rd Brigade, 3rd Division, XXI Corps, of the Army of the Cumberland. They remained on duty at Murfreesboro until June 23, participating in the Tullahoma Campaign through July 7. The regiment marched to McMinnville, and remained on duty there until August 16.
The regiment participated in the Chickamauga Campaign August 16 through September 22, including the Battle of Chickamauga September 19–20, where they suffered significant losses. The regiment was part of the siege of Chattanooga September 24 through November 23, helping to force the reopening of the Tennessee River October 26 through October 29.
The regiment was then attached to the 2nd Brigade, 1st Division, IV Corps of the Army of the Cumberland, participating in the Chattanooga-Ringgold Campaign November 23 through November 27. That participation included the Battle of Orchard Knob on November 23, the Battle of Lookout Mountain November 23-24, Missionary Ridge on November 25, Pigeon Hills on November 26, then Ringgold Gap and Taylor’s Ridge on November 27.
The regiment camped at Shellmound until February, when they were ordered to Dalton, Georgia, arriving there on February 22. They participated in the fighting at Tunnel Hill, Buzzard’s Roost Gap, and Rocky Faced Ridge February 23 through February 25.
Following the fighting, the regiment bivouacked at Cleveland, Georgia through the end of April. On May 1, they were ordered to advance on Atlanta. They again fought at Tunnel Hill (May 6–7), Buzzard’s Roost Gap (May 8–9), Dalton (May 9–13), and Resaca (May 14–15). They fought their way through Kingston (May 18–19), Cassville (May 19) and then advanced on Dallas (May 22–25). They fought at Pumpkin Vine Creek, Dallas, New Hope Church and Allatoona Hills May 25 through June 5.
The regiment was then transferred to the 2nd Brigade, 1st Division, XXIII Corps of the Army of the Ohio. The regiment saw nearly constant action in operations near Marietta and Kennesaw Mountain, including Pine Hill (June 11–14), Lost Mountain (June 15–17), Muddy Creek (June 17), Noyes Creek (June 19), and Kolb’s Farm (June 22). The regiment participated in the assault on Kennesaw Mountain (June 27), saw action at Nickajack Creek (July 2), Ruff’s Mills (July 3–4), the Chattahoochie River (July 5–17), and Decatur (July 19).
The regiment was then attached to the 1st Brigade, 2nd Division, XXIII Corps, of the Army of the Ohio.
They participated in the Siege of Atlanta beginning July 22, seeing action at Utoy Creek (August 5–7), the Battle of Jonesboro (August 31 through September 1), and Lovejoy’s Station (September 2–60.
The pursued General John Bell Hood’s army into Alabama (October 3–26) and then participated in the Nashville Campaign, fighting at Duck River (November 24–27), the Battle of Franklin (November 30), and the Battle of Nashville (December 15–16). The regiment pursued Hood’s troops to the Tennessee River December 17 through December 28.
The 99th Ohio Volunteer Infantry Regiment ceased to exist on December 31, 1864, when it was consolidated with the 50th Ohio Volunteer Infantry. The regiment lost a total of 342 men; 4 officers and 80 enlisted men were killed or mortally wounded; and, 2 officers and 256 enlisted men died of disease.