|May||Organized at Corinth under Colonel Richard Griffith, Lieutenant Colonel William H. Taylor and Major John R. Dickens|
|Sent to Union City, Tennessee|
|July 16||Started for Lynchburg, Virginia|
|July||Arrived at Manassas too late for the battle and was assigned to Rodes’ Brigade.|
|Quartered near Centerville.|
|November 2||Colonel Griffith was promoted to brigadier general and given command of a brigade of Mississippi regiments that did not include the 12th. Lieutenant Colonel Taylor was promoted to colonel.|
|Spring||The regiment moved with Rodes’ Brigade to the Virginia Peninsula as part of General D.H. Hill’s Division. It listed “1,013 men, rank and file” on its muster roll, one of the largest regiments in the army.|
|May 1||Major Dickens transferred into the Army of Mississippi. William H. Lilly was promoted to major|
Battle of Seven Pines
Supported by the 5th Alabama, charged under heavy fire through abatis and captured Federal rifle pits. General Rodes mentioned the gallantry of Colonel Taylor, Captain Hastings, and Sergeant Robert Hall. The regiment lost 41 killed and 152 wounded. Captain James Bell was wounded.
|June 9||The regiment was transferred to the newly created Second Mississippi Brigade commanded by Brigadier General Winfield S. Featherston. It was brigaded with the 19th Mississippi and Taylor’s 2nd Mississipi Battalion in Longstreet’s Division.|
Gaines’ Mill or Cold Harbor
Commanded by Major W.H. Lilly, the regiment charged up a hill and through a forest to capture a Federal battery. Major Lilly was wounded, and Captain Thomas took over the regiment.
Glendale, or Frasier’s Farm
The regiment lost 34 men killed, 186 wounded and 5 missing in the fighting from the 27th to the 30th. Captains J. Dobbins and J.E. Vawter and Lieutenants T.W. Crump and W.B. Thomas were killed.
Second Manassas Campaign
Longstreet as senior major general was given the command of several divisions in the army, so his own division was split up between its two senior brigadiers. The 12th Mississippi along with the rest of Featherston’s Brigade were assigned to Wilcox’s Division.
The regiment threw back a Federal cavalry charge at this ford across the Rappahannock. General Wilcox commended the regiment for its gallantry, along with the 16th Mississippi.
Second Battle of Manassas
The regiment fought near the stone house.
The three brigades of Wilcox’s temporary division were assigned to Anderson’s Division.The division was part of Longstreet’s Command but was temporarily assigned to Jackson’s Command for the attack on Harper’s Ferry. It returned to Longstreet’s control when it reached Sharpsburg on the morning of the battle.
|Capture of Harpers Ferry|
The regiment was commanded by Colonel William H. Taylor. It crossed the Potomac at daybreak and reached the field in mid-morning. Took position on the Piper farm south of the Sunken Road and supported D.H. Hill’s Division until forced to fall back to the protection of the stone walls along the Hagerstown Pike. The regiment lost 6 killed and 53 wounded.
|November||After the Confederate Congress authorized the creation of army corps Longstreet’s Command became the First Corps. The 12th Mississippi and Featherston’s Brigade were assigned to Anderson’s Division in the 1st Corps.|
The regiment spent three days and nights in the line of battle under artilery fire, losing eight men wounded
|January||Carnot Posey was promotd to brigadier general and took over the brigade from Featherston. The brigade remained with Anderson’s Division in the 1st Corps.|
|February||Posted to United States Ford on the Rappahannick River|
|February 19||Major William H. Lilly killed in an accident|
|April||After Union troops began to cross the Rappahannock the regiment was withdrawn to the junction of the old Mine and Plank Roads near Chancellorsville, where they put up breastworks|
The regiment took part in Jackson’s flank march and attack. The regiment served as skirmishers for the brigade in the attack. Lieutenant Colonel Merry B. Harris commanded the regiment and was badly wounded. Major Samuel B. Thomas then took over command. On May 2nd the regiment attacked near Catharine Furnace, and on May 3 it attacked and drove Union forces from their works around Chancellorsville. The loss for the regiment in the Chancellorsville fighting was 3 men killed, 38 wounded, and 23 missing.
|May 4||Marched to Fredericksburg to attack Sedgwick’s forces there.|
|June||The regiment with Posey’s Brigade and Anderson’s Division was transferred to the newly created Third Corps.|
The regiment brought 305 men to the field. It was in reserve throughout the battle and lost only lost 1 man killed and 6 wounded.
From the brigade monument on the Gettysburg battlefield:
July 2. Arrived and took position here in the morning. Through some misunderstanding of orders instead of the Brigade advancing in compact ranks in support of the troops on its right in their assault on the Union lines the regiments were ordered forward at different times. Deployed as skirmishers and fighting in detachments they pushed back the Union outposts and drove some artillerists for awhile from their guns but did not join in the attack upon the Union position on Cemetery Ridge.
July 3. Was held in reserve here supporting artillery in its front.
|August-September||Near Orange Court House|
Battle of Bristoe Station
General Posey was mortally wounded, and Colonel Harris of the 19th Mississippi took over the brigade.
|November||Mine Run campaign|
|March 19||Captain James R. Bell of Company I was promoted to Major.|
The regiment advanced from the Plank Road to protect the division’s flank, then defended their position
|May 8||Forced march to Spotsylvania Court House|
Po River bridge
The regiment lost 13 men killed, 32 wounded and 13 missing in the fighting since the beginning of the month.
The regiment counterattacked at the Bloody Angle to regain the Confederate line inside the Muleshoe and held it all day until a relief line could be constructed in the rear. Lieutenant Bow was killed.
|May 24||Battle of the North Anna|
Captain Hannegan was mortally wounded by an artillery shell
|June 18||Moved into Petersburg defences|
|June 24 – 25||Weldon Railroad
Colonel Harris was shot in the head by a sharpshooter. He was permanently diabled and would die from complications of his wounds in August of 1865.
|June||The regiment lost 10 men killed, 20 wounded and 8 missing during the month of June.|
|August 18||Battle of Darbytown Road|
Battle of Reams’ Station on the Weldon Railroad
The regiment took and occupied Federal earthworks but were counterattacked and overwhelmed by the Federal Iron Brigade, losing many captured. Colonel Samuel Thomas, Major Bell, Adjutant Howard McCaleb and Captain Joseph Johnson were wounded and captured.
|August||The regiment lost 4 men killed, 26 wounded and 63 missing or capturedin August.|
Battle of Hatcher’s Run
|April 2||The survivors of the regiment were under the command of Lt. Colonel James H. Duncan of the 19th Mississippi. They were ordered to leave the Swift Run line and double timed to Petersburg to fill a gap in General Wilcox’s line, then placed in Battery Gregg to slow the Federal breakthrough. After a desperate defense that caused high Federal casualties the battery was taken and its entire garrison killed or captured.|
Appomattox Court House
One Assistant-Surgeon and 53 enlsted men surrendered under Sergeant William Brown of Company K.