Alonso De León, the younger, Spanish explorer and governor, third son of Alonso De León and Josefa González, was born in Cadereyta, Nuevo León, in 1639 or 1640. At the age of ten he was sent to Spain, where he enrolled in school and prepared for a naval career. He joined the Spanish navy in 1657, but his service as a naval cadet was brief, for he had returned to Nuevo León by 1660. Over the next two decades he led a series of entradas that traversed the northeast coast of New Spain as well as the banks of the Río de San Juan. By the 1680s De León had become a seasoned outdoorsman and successful entrepreneur. In 1682 he petitioned the viceroy of New Spain for a franchise to work salt deposits along the Río de San Juan, open trade with neighboring settlements, and search for mines. Those efforts netted a fifteen-year concession. When news that the French had founded a settlement on the northern Gulf Coast reached New Spain in the mid-1680s, De León was a logical choice to lead overland efforts aimed at finding the foreign interlopers and extirpating their colony.
In all, he led four expeditions between 1686 and 1689. His initial reconnaissance followed the Río de San Juan to its confluence with the Rio Grande. After striking the larger river, Don Alonso marched along the right bank to the coast and then turned southward toward the Río de las Palmas (the Río Soto la Marina). This effort yielded no conclusive evidence that Frenchmen had visited the region. His second expedition set out in February 1687. This entrada forded the Rio Grande, probably near the site of present Roma-Los Saenz, and followed the left bank to the coast. De León then marched up the Texas coast to the environs of Baffin Bay but again found no evidence of Frenchmen. The third expedition, launched in May 1688, was in response to news that a white man dwelled among Indians in a ranchería (temporary settlement) to the north of the Rio Grande. That effort resulted in the capture of Jean Jarry, a naked, aged, and confused Frenchman. The fourth expedition left Coahuila on March 27, 1689, with a force of 114 men, including chaplain Damián Massanet, soldiers, servants, muleteers, and the French prisoner, Jarry. On April 22 De León and his party discovered the ruins of the French settlement, Fort St. Louis, on the banks of Garcitas Creek.
In 1687 De Léon became governor of Coahuila. Three years later he and Massanet cooperated in founding the first Spanish mission in East Texas, San Francisco de los Tejas, at a site in the environs of present Augusta, Texas. De León, an honest soldier and an early pathfinder in Spanish Texas, left the future Lone Star State for the last time in July 1690. He is credited with being an early advocate for the establishment of missions along the frontier, and he blazed much of the Old San Antonio Road on his expeditions. He returned to Coahuila and died there on March 20, 1691. His survivors included his wife, Agustina Cantú, four sons, and two daughters. His descendants still reside in the Mexican state of Nuevo León.