Thomas B. Monroe, Jr., was the fourth son of Judge Thomas Bell Monroe, Sr. He was born on July 3, 1833, at Frankfort. He was educated at the University of Louisiana while his father was a professor there. In 1849, he settled in Lexington and began the practice of law. In 1856, he became editor of the Democratic newspaper, the "Kentucky Statesman". In 1859, he was elected mayor of Lexington. In 1860, he endorsed states’ rights and the presidential candidacy of fellow Kentuckian John C. Breckinridge.
Monroe was appointed Secretary of State in September 1859 in the administration of Governor Beriah Magoffin, becoming the youngest person to ever occupy the office. He served as Secretary of State until October 5, 1861, when he resigned his office to serve as Major of the Fourth Kentucky Confederate Infantry. He was killed at an engagement at Burnsville on April 6, 1862, and was buried on the field of battle by Union soldiers. After the war, he was reinterred in the Frankfort Cemetery next to his brother Captain Ben Monroe.
Monroe married Elizabeth C. Grier of Philadelphia on November 15, 1859. She was the daughter of Justice Robert C. Grier of the United States Supreme Court. The couple had one son.
"Biographical Encyclopedia", (1878), page 548;
Ed Porter Thompson, "History of the Orphan Brigade", (1898), pages 458-66.