Seguín's military career began in 1835. In the spring he responded to the Federalist state governor's call for support against the Centralist opposition by leading a militia company to Monclova. After the battle of Gonzales in October 1835, Stephen F. Austin granted a captain's commission to Seguín, who raised a company of thirty-seven. His company was involved in the fall of 1835 in scouting and supply operations for the revolutionary army, and on December 5 it participated in the assault on Gen. Martín Perfecto de Cos's army at San Antonio. Seguín entered the Alamo with the other Texan military when Antonio López de Santa Anna's army arrived, but was sent out as a courier. Upon reaching Gonzales he organized a company that functioned as the rear guard of Sam Houston's army, was the only Tejano unit to fight at the battle of San Jacinto, and afterward observed the Mexican army's retreat. Seguín accepted the Mexican surrender of San Antonio on June 4, 1836, and served as the city's military commander through the fall of 1837; during this time he directed burial services for the remains of the Alamo dead. He resigned his commission upon election to the Texas Senate at the end of the year.
He spent six years in Mexico and then attempted to reestablish himself in Texas. While living in Mexico he participated, according to him under duress, in Gen. Adrián Woll's invasion of Texas in September 1842. Afterward his company served as a frontier defense unit, protecting the Rio Grande crossings and fighting Indians. During the Mexican War his company saw action against United States forces. At the end of the war he decided to return to Texas despite the consequences. He settled on land adjacent to his father's ranch in what is now Wilson County. During the 1850s he became involved in local politics and served as a Bexar County constable and an election-precinct chairman. His business dealings took him back to Mexico on occasion, and at the end of the 1860s, after a brief tenure as Wilson county judge, Seguín retired to Nuevo Laredo, where his son Santiago had established himself. He died there on August 27, 1890. His remains were returned to Texas in 1974 and buried at Seguin, the town named in his honor, during ceremonies on July 4, 1976.
29° 33.704, -097° 58.251
Juan N. Seguin Memorial Plaza