From 1686 to 1690, General Alonso de León led five military expeditions from Northern New Spain into modern-day Texas in search of French intruders who had breached Spanish sovereignty and settled on lands claimed by the Spanish Crown. His first two exploratory journeys were unsuccessful, but on the third expedition, he discovered a Frenchman living among Coahuiltec Indians across the Río Grande. In 1689, the fourth expedition finally led to the discovery of La Salle’s ill-fated colony and fort on the Texas Coast and to the repatriation of two of the French survivors. On his fifth and final expedition, De León established the first Spanish mission among the Hasinai Indians of East Texas and rescued several French children who had been abducted by the Karankawa. Through archival research, I have identified sixteen manuscript copies of De León’s meticulously kept expedition diaries. These documents form a distinct corpus and hold major importance for early Texas scholarship. Several of these manuscripts, but not all, have been known to historians and have been addressed in the literature. However, never before have all sixteen manuscripts been studied as an interconnected body of work and submitted to philological treatment. In this interdisciplinary study, I transcribe, translate, and analyze the diaries from two different perspectives: linguistic and historical. The linguistic analysis examines the most salient phonological, morphosyntactic, and lexical phenomena attested in the documents. This synchronic study provides a snapshot of the Spanish language as it was used in Northern Mexico and Texas at the end of the 17th century. An in-depth examination discovers both conservative traits and linguistic innovations and contributes to the history of American Spanish. The historical analysis reveals that frequent misreadings, misinterpretations, and mistranslations of the Spanish source documents have led to substantial factual errors which have misinformed historical interpretation for more than a century. Thus, I have produced new, faithful, annotated English translations based on the manuscript archetypes to address historical misconceptions and present a more accurate interpretation of the historical events as they actually occurred.