Thomas was born in Cabarrus County, North Carolina on May 28, 1810 and grew up on the family plantation in Columbia, Tennessee.
His older brother was James Houston Thomas (1808–1876), a member of the U.S. House of Representatives from Tennessee's 6th district.
[3] From 1840 to 1841 he was assistant professor of geography, history and ethics at West Point and from 1842 to 1845 he was commandant of cadets and instructor of infantry tactics.
On July 23, 1846, he became colonel of the 4th New York regiment which had been raised for the war with Mexico, but was not mustered into service.
[3] Thomas was chief engineer of New York state in 1853 to 1854 and from April 19, 1853 to January 14, 1854 was advocate of the United States in London, England under the convention of February 8, 1853 with Great Britain for the adjustment of American claims.