Assigned to San Xavier del Bac in 1768, he explored the Gila and Colorado River Valleys, both down the Colorado to the Gulf of California and up it to the Grand Canyon and overland to the Hopi villages. He accompanied Anza part way in both 1774 and 1775-76. He and Juan Díaz established two mission churches on the Colorado at Yuma Crossing. They and Juan de Barreneche and José Moreno were all killed there during the Yuma uprising. His baptism and confirmation are certified as follows:
"Friar Pedro Alaman, Predicator and Guardian of the County of San Lorenzo de la Almumá de Río in the Province of Aragón, of the Order and Convent of Our Father Lord Francisco Diego, having gone to the village of Morata del Conde and observed the five books of the parochial church or of the said village, I found the baptismal entry of the following person in the book of the baptized in the year 1738 at folio 57: Francisco Tomás Hermenegildo Garcés In the parochial church of Morata del Conde on April 13, 1738 I, Sir Pedro Gerónimo Villalba, Rector, baptized a child who was born on the twelfth of the said month. He is the son of Juan Garcés and Antonia Maestro, legitimately married parishioners of this church and who presently live here and have a habitation is this village of Morata. I gave him the name of Francisco Tomás Hermenegildo. He is the grandson of Juan Garcés and Josefa Mercado, and of Juan Maestro and Antonia Cabeza. His godfather was Sir Francisco Garcés who entered into the spiritual relationship and obligation to teach our doctrine to the said baptized boy in case of failure on the part of the parents. Sir Pedro Gerónimo Villalba, Rector At the same time, in the book of the confirmed I found the following: During the holy visit celebrated by the Most Illustrious Lord Bishop of Tarazona on October 2, 1739 Francisco Garcés, son of Juan Garcés and Antonia Maestro, was confirmed. The godfather was Sir Domingo Garcés. For the truth of which I made the present certification in the above mentioned village of Morata del Conde on June 16, 1783 and signed it by my hand and sealed it with the seal of the said County of San Lorenzo de la Almumá. Fr. Pedro Alaman, Predicator and Guardian"