In 1822, Desloge's father, Firmin Rene Desloge, came to America from France to work with his uncle Jean Ferdinand Rozier from Ste. Genevieve, Missouri. Born in 1843, the young Desloge received his early education in the public schools at Potosi, where the family businesses included fur trading, distilling, and mining. He then attended St. Louis University and later the commercial school of Bryant & Stratton in St. Louis, Missouri. He was trained to mercantile pursuits, beginning at an early age as a clerk for the firm of John B. Valle & Co. of St. Louis. In 1867, he began mining operations near Potosi. When lead mining was in its infancy in St. Francois County, Missouri, he prospected lands in that county adjacent to those of the St. Joseph Lead Company, and finally purchased and erected smelting works for the corporation known as the Desloge Lead Company. Desloge built a connection with the St. Joseph Lead Company—the first railroad to penetrate the disseminated lead field of St. Francois County. In 1887, the two companies merged to create what was probably the era's greatest lead-mining and smelting company. In 1889, he acquired from the Bogy Lead Mining Company one of the oldest mining properties in Missouri, and after demonstrating that there were valuable deposits of disseminated lead on these lands, folded them into the new Desloge Consolidated Lead Company. The town built to support the mines is now known as Desloge, Missouri. The use of the new diamond drill and the 1893 arrival of a branch from the Mississippi River & Bonne Terre Railroad allowed the already-successful lead mining operations to expand. A grandson of one of Desloge's friends and business colleagues, Harry Cantwell, Sr, said, "They say that grandfather and Desloge were riding in a surrey one day trying to decide where to sink a shaft. Desloge spit off one side of the surrey and said there was where they would sink the shaft. Grandfather didn't agree with the location of the spit and split with Desloge to form his own company." Desloge soon sank a shaft and struck the same main vein and deposits as that of those he had worked at Bonne Terre before the fire.
In 1930, a $1 million bequest from Desloge's estate built Firmin Desloge Hospital in St. Louis. Desloge's wife Lydia gave another $100,000 to build the adjacent Desloge Chapel.
Death
Firmin V. Desloge died December 18, 1929. His estate was settled in 1932, valued at more than $52 million. He was one of the wealthiest men of that era, alongside W. K. Vanderbilt and A. W. Mellon, but only half as wealthy as the Astors.
