[1] The St. Mary's Cannoneers battery was accepted into Confederate service on 7 October 1861 at Franklin, Louisiana, with Florian O. Cornay as captain.

Foot along with B and D Companies of the 1st Louisiana Heavy Artillery were assigned to man the Fort Jackson water battery.

The warships of the Union fleet began moving up the Mississippi River at 3:30 am on 24 April and despite heavy fire, successfully passed the forts.

[2] Union Major General Nathaniel P. Banks concentrated two and a half infantry divisions at Brashear City on 9 April, and began advancing up the Atchafalaya River and Bayou Teche.

However, news of Major General Ulysses S. Grant's Vicksburg campaign caused Banks to quit the west side of the Mississippi River on 23 May and start operations against Port Hudson.

[6] On 3 June 1863, the St. Mary's Cannoneers fought with the Union gunboat USS Estrella on the Atchafalaya and forced it to retreat.

From July 1863 to March 1864, the battery joined Brigadier General Alfred Mouton's brigade in its operations in south Louisiana, but there was little fighting.

On 8 December, one section of the St. Mary's Cannoneers damaged the river transport Von Phul and clashed with the monitor USS Neosho.

[6] At the Battle of Mansfield on 8 April 1864, Cornay's battery was placed in the front line between the divisions of Mouton and Major General John George Walker.

In the Action of 26–27 April 1864, Cornay's guns hit the gunboat USS Cricket (with Porter aboard) 38 times, killing 12 crewmen and wounding 19.

The battery arrived on the field while covered by Colonel Xavier Debray's Texas cavalry brigade and began firing at 7:30 am.

At about 10:00 am, the two howitzers under Lieutenant Oscar D. Berwick were shifted to the extreme left flank in order to engage a force of Union troops that were massing in that direction.

While assigned to Brigadier General Camille de Polignac's division, it marched through north Louisiana and south Arkansas later in the year.