Francisco Álvarez Barreiro traveled from Spain to New Spain in the company of Viceroy Marqués de Valero (1716–22). Shortly after his arrival in the New World, Álvarez Barreiro was appointed military engineer for the expedition of Governor Martín de Alarcón, charged with founding religious, military, and civilian settlements on the San Antonio River and the resupply of missions in East Texas. According to his own testimony he assisted in the construction of the chapel for San Antonio de Valero Mission.
In 1720 Álvarez Barreiro was apparently obliged to return to Spain under a general order, which stated that Spaniards with wives in Spain should return there. However, by 1724 he was back in Mexico. In that year he began his most important work as surveyor, map maker, and experienced engineer for a massive inspection of northern New Spain (1724–28), carried out by Brigadier General Pedro de Rivera y Villalón.
Álvarez Barreiro left the capital on November 21, 1724, on a trek that eventually covered nearly 7,000 miles. The inspection began at Zacatecas and progressed to all presidios in northern New Spain. Prolonged stopovers were often necessary in order for him to complete his surveys and maps. Rivera completed his tour of inspection in Texas in the latter months of 1728. In all, Álvarez drafted six maps, five of which are located in the Archivo General de Indias. From presidio La Bahía, he spent thirty-five days exploring the coast and land that lay between it and the Neches River. His efforts represented "the most comprehensive reconnaissance of the upper Texas coast yet achieved." The resulting map, entitled Plano, corographico é hidrographico, is preserved in the British Museum. While it repeats a few errors, such as the misconception that the Guadalupe River flows into Matagorda Bay, its accuracy in other respects is surprising. Contained within it are the configuration of the coast, some rivercourses, and Indian villages and Spanish settlements.
In Álvarez Barreiro's final landmark survey, he recorded logging 363 leagues, or about 944 miles. He rejoined the Rivera inspection caravan at San Juan Bautista on December 23, 1728. There he received a new assignment, after which he disappears from known historical records.