After serving in the Missouri General Assembly from 1842 to 1843, he fought as a captain in the United States Army for fourteen months during the Mexican–American War, beginning in 1846.

After the outbreak of the American Civil War in April 1861, Slack, who held pro-slavery views, supported the Confederate cause.

On March 7, 1862, during the Battle of Pea Ridge, Slack suffered another wound that was close to the injury he had received at Wilson's Creek.

After returning to Columbia in 1839, he was admitted to the bar and relocated to Chillicothe[1] where he opened a law office, as there were more opportunities for new lawyers in that town.

[4] Engaged in the fighting that took place in the Santa Fe area, Slack's conduct at the Battle of Embudo Pass in January 1847 gained praise from Price, and Slack's men blocked the enemy's retreat route in the Siege of Pueblo de Taos.

[10] Tensions grew in Missouri, with Governor Claiborne Fox Jackson mobilizing pro-secession militia to oppose Union Army forces.

[16] Slack moved his men to the Lexington area, but by that time the MSG had abandoned Jefferson City, the state capital, and had been defeated in the Battle of Boonville.

[2] Jackson had gathered together MSG troops near Lamar, and in early July began moving to join Price in southern Missouri.

Jackson formed his troops – the divisions of Slack, Rains, John B. Clark Sr., and Mosby Monroe Parsons – in preparation to defend against an attack.

[14] Slack's infantry was most heavily engaged during the later stages of the battle, when the retreating Union soldiers were being pursued near the town.

[22] The MSG then gathered at Cowskin Prairie in McDonald County, but it moved to Cassville to unite with McCulloch's command.

[14] Late on August 9, Slack and several other officers supported Price in pushing McCulloch to order an attack against Springfield, which was scheduled to begin the next morning.

[25] However, Lyon struck the Confederate camp in a surprise attack on the morning of August 10, bringing on the Battle of Wilson's Creek.

Slack's cavalry, under the command of Colonel Benjamin A. Rives, made a brief stand that allowed Confederate troops elsewhere time to reorganize, but was driven back.

[14] This infantry was later engaged in assaulting a position that Union troops had taken up on a prominence known as Bloody Hill, at one point holding the right of the Confederate line.

[29] McCulloch's post-battle report praised Slack,[30] although the latter suffered a bad hip wound while leading an assault.

[32] In January 1862, Price abandoned Missouri and fell back into Arkansas, having been pressured by Union forces commanded by Brigadier General Samuel R. Curtis.

Major General Earl Van Dorn was placed in command of Confederate forces in the Trans-Mississippi, and he began a campaign to retake Missouri.

[38] The historians William L. Shea and Earl J. Hess report that Slack was competent and popular, but had a tendency to be reckless when it came to his personal safety.

[40] Burchett describes Slack as "of much more than ordinary ability, cool and clearheaded, and a model of soldierly bearing".