He was a brigadier general in the Confederate Army during the Civil War, a three-term member of the United States House of Representatives from 1881 to 1887, and Secretary of the Senate from 1893 to 1900.

With North Carolina's secession and the outbreak of the Civil War in early 1861, Cox raised and outfitted the "Ellis Artillery Company".

While temporarily in command of Ramseur's Brigade because the general was on leave to get married, Cox was wounded in the face and right shoulder early in the battle of Kelly's Ford on November 7, 1863.

[4] He fought with distinction at the battles of the Wilderness and Spotsylvania Court House, being personally commended by General Robert E. Lee for bravery in fighting on May 12.

Returning to the Army of Northern Virginia, Cox served in the trench defenses during the Siege of Petersburg, including the counterattack of Confederate forces on the Union's Fort Stedman.

In the House, he championed civil service reform, a stance which alienated some prominent Democrats, leading to him losing the party nomination for re-election in 1886.

[5] He remarried and retired to his plantation in Edgecombe County, but was appointed Secretary of the U.S. Senate to replace former Union army general Anson G. McCook in 1893.