Crittenden had brief duty as a private in the Kentucky Infantry in 1836 when war with Mexico seemed possible. During the Mexican War a decade later, as an aide on the staff of General Zachary Taylor, Crittenden was selected to carry news of the victory at Buena Vista to President James K. Polk. Crittenden then served as lieutenant colonel of the Third Kentucky Infantry in which John C. Breckinridge was major. The regiment joined General Winfield Scott's army after Mexico City was captured. Crittenden's standing with Taylor was enhanced in 1848 when his father abandoned his friend Henry Clay and helped General Taylor win the presidency. Thomas Crittenden then secured the lucrative consulship at Liverpool in 1849. Returning to Kentucky in 1853, he practiced law and engaged in business undertakings. He was appointed colonel of a volunteer Kentucky regiment in 1858, but the "Mormon War" ended before the troops could be sent to Utah.
Lowell H. Harrison, "Crittenden, Thomas Leonidas," American National Biography Online, February 2000, http://www.anb.org/articles/04/04-00281.html.