During the Mexican–American War he served on the staff of Major General J. Pinckney Henderson and fought in the Battle of Monterrey.

[3] Among Clark's first actions was to order the surrender of all fire arms and ammunition from private merchants to the state.

Fehrenbach, pg 353) After losing the governor's race by 124 votes to Francis Lubbock, Clark became a colonel in the Texas militia during the American Civil War.

In 1863 he joined the Confederate States Army and was commissioned colonel of the 14th Texas Infantry Regiment.

He fled briefly to Mexico at the end of the American Civil War, and returned home to Marshall, Texas.