Deas was born in Camden, South Carolina, a son of James Sutherland Deas and Margaret Rebecca (Chesnut) Deas. He was a cousin of future fellow Confederate general James Chesnut, Jr. He was educated in South Carolina and in Caudebec, France. In 1835, his family moved to Mobile, Alabama, where he initially engaged in the mercantile business. He served in the army during the Mexican-American War. After the war, he became a prominent cotton broker in Alabama and amassed a significant fortune from his speculation. He married Helen Gaines Lyon on May 16, 1853.
After recovering. he commanded a brigade under General Braxton Bragg in the Kentucky Campaign. His command consisted of the 19th Alabama, 22nd Alabama, 25th Alabama, 39th Alabama, 50th Alabama, 17th Alabama Battalion Sharpshooters, and Dent's Alabama Battery. He was appointed as a brigadier general on December 13, 1862, and fought at the Battle of Chickamauga, where his men routed the Union division of Philip H. Sheridan and killed Brig. Gen. William H. Lytle. They also captured seventeen artillery pieces.[2]