After admission to the Virginia bar, Montague established his legal practice in Middlesex County. When he earned enough money to buy a plantation, he began farming using enslaved labor, in addition to his legal practice. He became politically active as a
Democrat campaigning for
James Polk in the Presidential Campaign of 1844. Voters in
Mathews and
Middlesex Counties elected him to represent them in the
Virginia House of Delegates in 1850, but two years later he instead ran for (and was elected) Commonwealth Attorney, which position he held until 1860, having won re-election several times. Voters from Mathews and Middlesex Counties elected him as a delegate to the
Virginia Secession Convention of 1861, and he became the Convention's President after
John Janney resigned on November 16, 1861. He was an ardent defender of slavery and secessionist, and was physically in the president's chair when the ordinance of secession passed. Briefly during the
American Civil War, Montague led both legislative bodies meeting in Richmond. Elected Virginia's Lieutenant Governor in 1861 (and polling 5000 more votes than John Lechter who led the ticket and was elected Governor of Virginia), Montague served ex officio as President of the Virginia Senate beginning in 1861 until December 21, 1863, when he became one of Virginia's delegates to the
Confederate States Congress (1864-1865). His youngest (and favorite) brother, Andrew Jackson Montague, while a cadet at the Virginia Military Institute, volunteered to defend Richmond, and died during the Battle of Gaines' Mill in 1862. After the war, in 1871, Middlesex County voters again elected him to represent them in the House of Delegates. He represented Middlesex County for one term, before fellow legislators elected him in 1875 to an eight-year term as a judge of the 8th Circuit Court, which he died before finishing. ==Death and legacy==