Nicolás de Finiels was born in France about 1767. He traveled to America with the French expeditionary force and served as a captain of engineers during the American Revolution. In 1797, armed with a passport from the Spanish minister to the United States, he went from Philadelphia to St. Louis to help with fortifications of upper Louisiana during the period when Spain feared a British attack from Canada. Despite jurisdictional disputes with the Flemish engineer, Luis de Vandembenden, and Col. Carlos Howard, Finiels drew several plans of the Missouri River and environs. Ordered to leave Louisiana and the Spanish service in 1798 by the minister of war in Spain, he went to New Orleans, where his patron, Governor-general Manuel Gayoso de Lemos, continued to employ him as an engineer and draftsman in drawing plans of Baton Rouge and New Orleans. A royal order of June 19, 1799, reinstated Finiels in the service. He was a valuable member of the staff of the governor-general, the Marqués de Casa Calvo. In addition to his skill in cartography, Finiels was an inventor of surveying instruments. Casa Calvo chose him as chief surveyor of the 1805 expedition to western Louisiana and Texas.
The party left New Orleans in October, ascended by canoe up the Bayou de La Fourche de Chetimachas, descended the Atchafalaya River, and followed the coast toward Texas. They charted the Gulf Coast and in particular the Mermentau, Calcasieu, Sabine, and Trinity rivers. The expedition employed the galiot Vigilante to explore San Bernardo Bay, from which point they trekked overland to Atascosito, near the Trinity River, and thence to San Antonio and Nacogdoches. After three weeks of examining mission records to determine the location of the mission and presidio of San Miguel de Linares de los Adaes, the expedition examined the region toward Arroyo Hondo, some 2½ leagues west of Natchitoches and the boundary between Louisiana and Texas. Upon the return of the expedition to New Orleans in 1806, Finiels and other Spanish officers were expelled from Louisiana by Governor William C. C. Claiborne. Lt. Col. Finiels was attached to the Pensacola garrison, where he soon became engineer in chief of West Florida, a post he held until 1819. He supervised fortifications at Pensacola, San Carlos de Barrancas, and Mobile, and in 1818, as a member of the Security and Vigilance Committee, was wounded when Andrew Jackson captured Pensacola. Finiels was twice married. He and his second wife, Mariana (Rivier), a native of Pensacola, had four sons, all of whom served in the Spanish army.