He entered the Confederate army as a lieutenant and was promoted successively to the rank of captain and then major in the 6th Missouri Infantry.

Primarily serving in Missouri and Arkansas under Thomas C. Hindman, he was rewarded with a commission as a brigadier general on March 6, 1864.

He then fought in the Trans-Mississippi Theater under John S. Marmaduke and Jo Shelby, including Price's Raid.

After the war, he resumed his law practice in Fayette and was elected as a Democrat to the Forty-third and to the four succeeding Congresses, serving from 1873 until 1883.

Clark was an unsuccessful candidate for renomination in 1882, but stayed in Washington, D.C. as the Clerk of the House of Representatives from 1883 until 1889, when he retired from politics.