Clark served in the Texas Annexation Convention and two terms as a state representative in the Texas Legislature before fighting in the Mexican-American War. When the war ended, he served as secretary of state under Governor Elisha M. Pease and as lieutenant governor under Sam Houston. When Sam Houston refused to take an oath of allegiance to the Confederacy, the new Texas Legislature declared the governors office vacant and Clark became governor.

After losing the governor's race in the autumn of 1861 by 124 votes to Francis Lubbock, Clark organized and became commander of the 14th Texas Infantry as a colonel. Clark was wounded in the leg while leading an attack at the battle of Pleasant Hill and subsequently discharged from the army. He fled briefly to Mexico at the end of the American Civil War, but returned home to Marshall, Texas, where he died May 4, 1880. He is burried in Marshall City Cemetery.