Mariano Varela, a creole Spaniard born in Mexico about 1770, was a captain at Guajoquilla (present-day Jiménez) in western Coahuila in May 1807, when Zebulon M. Pike arrived there from Chihuahua. Pike, being escorted out of Spanish territory, traveled thence with Varela to Presidio del Rio Grande (San Juan Bautista), where Varela assumed command. Pike found him to be "a gentleman in his manners-generous and frank; and I believe a good soldier." Varela, by Pike's account, had begun his military career as a cadet some twenty years previously at Guajoquilla. Captain Varela had won, "by his extraordinary merits," advancement to his present rank, which he considered "his ultimate promotion." The tide of revolution was to remove such a limitation. When the entourage arrived at Presidio del Rio Grande, Varela found his new post, which he had not seen previously, somewhat less attractive than the descriptions he had been given. In August 1808 he was summoned to San Antonio de Béxar to a war council convened by Texas governor Manuel Antonio Cordero y Bustamante. The matter to be dealt with was the visit of Napoleon's agent Octaviano d'Alvimar. By June 1809 Varela had been assigned to the staff of Bernardo de Bonavía y Zapata, the newly appointed military commander of Texas. He served as secretary of the juntas called by Bonavía to deal with economic reforms.
In March 1810, Texas governor Manuel María de Salcedo appointed Varela, by then serving temporarily as lieutenant governor, to serve also as commander of presidial troops and head of the political department. Varela assisted Bonavía in a reshuffle of command posts in the province before Nemesio Salcedo (commandant of Provincias Internas) ordered him to resume command at San Juan Bautista. Although the order was issued in May 1810, Bonavía delayed its execution until Governor Salcedo returned from East Texas. In the meantime, in response to Napoleon's overthrow of King Ferdinand VII, Lieutenant Governor Varela assembled royal officers and clergymen at Bonavía's residence to pledge their loyalty to the deposed monarch. Back at the Rio Grande post by August, he was asked by Governor Salcedo to check the suspected horse-stealing of Lipan Apaches encamped in his region. He replied that he lacked sufficient manpower to carry out the task. Varela's stay at San Juan Bautista evidently was short, for by January 1811 the community was dominated by insurgents, the presidio commanded by Antonio Griego, a rebel sympathizer. The record of Varela's activities during the early stages of the insurgency have not emerged, but he seems to have remained a staunch loyalist. Having been summoned back to Texas, he was serving as commandant of the cavalry in the province when he was elevated to the Texas governorship on July 20, 1815. One of a succession of men who filled the office between Salcedo and Antonio María Martínez, he served until ill health forced his retirement approximately one year later. Varela is last heard of in the post-revolutionary era. On August 14, 1824, he was installed as a deputy in the congress of Coahuila and Texas in Saltillo. He was present on March 11, 1827, when the congress was reconvened. The date and place of his death are not established.