Known as the "Louisiana Tigers", his brigade played a major role at the Battle of Gettysburg in July 1863, where they ascended Cemetery Hill in the darkness and overran several artillery batteries before finally being driven off for lack of support.

Returning to Louisiana, Hays became active in politics as a Whig and served as a Presidential Elector supporting Winfield Scott in 1852.

After fighting at the First Battle of Bull Run and Stonewall Jackson's Shenandoah Valley Campaign of 1862, he was shot in the shoulder and knocked unconscious by a shell burst at Port Republic.

At the Battle of Gettysburg in July 1863, after he had garrisoned the town, he directed his troops in a twilight assault on Cemetery Hill.

Reluctantly, and with casualties mounting, he ordered the remnant of his brigade to retire in the gathering darkness just as Federal reinforcements arrived to secure the heights.

He played a prominent role in the July 1866 New Orleans Riot, at one time deputizing nearly two hundred of his former soldiers who were now members/beneficiaries of the "Hays Brigade Relief Society."

Hays was removed from office in November by Federal government officials, at the insistence of influential former Union general Philip H. Sheridan.