From the Writings of New Orleans Historian Buddy Stall. On this Day April 9, 1682, Louisiana was discovered by French Explorer Robert Cavelier De La Salle. La Salle made the discovery out of Canada by way of the Mississippi River . In 1684, he was sent back by way of the Atlantic Ocean into the Gulf of Mexico to find the mouth of the Mississippi River. Unfortunately on March 20, 1687, he was assassinated by his disenchanted men in what is now Texas.