José Joaquín de Ecay (Eca y) Múzquiz, a Spanish army lieutenant, probably the son of José Antonio de Ecay Múzquiz, was stationed at Presidio de Santa Rosa del Sacramento in February 1750, when he was appointed to investigate the site of the proposed San Francisco Xavier Presidio. He was instructed to take charge of the garrison at San Xavier, to measure the lands and water of the San Xavier (San Gabriel) River, to take a census of the mission Indians, and to do anything else pertinent to the determination of the necessity for a presidio. In June 1750 Ecay Múzquiz surveyed the San Xavier River, and his favorable report ended the doubt standing in the way of final consent to the establishment of the presidio. He turned the garrison over to the newly appointed presidial commander, Felipe Rábago y Terán, on December 13, 1751.
On January 16, 1753, Ecay Múzquiz was appointed by the viceroy to assist Capt. Miguel de la Garza Falcón in the official inquiry into the murder of Fray Juan José Ganzábal of Nuestra Señora de la Candelaria Mission. After the death of his chief in August 1753 Ecay Múzquiz commanded the garrison until he was relieved by the new inspector, Pedro de Rábago y Terán, on August 11, 1754.
In 1757 Ecay Múzquiz was assigned as lieutenant of the garrison of San Luis de las Amarillas Presidio (San Sabá). As official courier for the commandant, Diego Ortiz Parrilla, he carried dispatches, conducted a prisoner to Mexico City, and escorted a new parish priest from Presidio de Santa Rosa. By 1759 he had succeeded Toribio de Urrutia as commandant of San Antonio de Béxar Presidio at San Antonio and was a member of the junta that recommended Ortiz Parrilla's campaign to punish the allied northern Indians responsible for the San Sabá Mission attack in March 1758. When Ortiz Parrilla departed for Mexico at the conclusion of that campaign, Ecay Múzquiz was the logical choice to fill the San Sabá post in his absence. An illness prevented his doing so. Consequently, Capt. Manuel Rodríguez of San Juan Bautista Presidio took temporary command at San Sabá, where he remained until the arrival of Ortiz Parrilla's successor in 1761.