Born in New Orleans, Clark moved to Texas in 1842 and set up a law practice. 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 E.M. Pease and as lieutenant governor under Sam Houston. When same Houston refused to take an oath of allegiance to the Confederacy, Clark became Governor.
After losing the governor's race by 124 votes to Francis Lubbock, Clark joined the 14th Texas Infantry as a colonel and was later promoted to brigadier general after being wounded in battle. He fled briefly to Mexico at the end of the American Civil War, but returned home to Marshall, Texas where he died in 1880. He grave is marked with a historical marker in the Marshall City Cemetery.