Upon his graduation from West Point, he was assigned to the 3rd artillery, served in garrison and as assistant instructor of infantry tactics and became 2nd lieutenant on December 1, 1835 and 1st lieutenant on June 30, 1837.[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. He was made captain on November 19, 1843 and resigned on May 28, 1846 to practice law in New York City, New York. 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. From November 1, 1855 to April 3, 1857 he was United States Assistant Secretary of State, serving under U.S. Secretary of StateWilliam L. Marcy and, later, Lewis Cass, in Washington, D.C.[4] He gained reputation by his report of the convention with Great Britain and by other state papers.
Addison Thomas (1845–1908), who married Alice Gridley Abbott (1850–1874), daughter of James Alexander Abbott.[7] After her death, he married Susan Cox (b. 1839), daughter of Rev. Samuel Hanson Cox.[1]
Ronald Thomas (1847–1923), who married Daisy Richards (1857–1900),[8] daughter of Dr. John Custis Richards, in 1881.[9] They lived at "The Roses" in Santa Barbara, California.[10] After her death in 1900, he married Julia Hayes Percy in 1902.[11]
Catherine Lorillard Thomas (1850–1934), who married Ernest Christian de La Haye, Viscount d'Anglemont (d. 1885)[12][13] in 1869.[1]