James Fleming Fagan first experienced life in the military during the Mexican-American War, through which he rose to the rank of lieutenant. After the war Fagan returned to Arkansas, and served one year with the state legislature.
At the outbreak of the Civil War Fagan was one of the first in the state to begin recruiting men, and was elected colonel of the 1st Arkansas Volunteers. He led the regiment through the battle of Shiloh, as well as in the siege of Corinth. After he was commissioned a brigadier general on September 12, 1862, Fagan transferred from his position under General Braxton Bragg to command in the Trans-Mississippi Department. Here he commanded during the battles of Cane Hill and Prairie Grove. Fagan is best known for his actions during the battle of Helena, as well as his participation in the defense of Little Rock. Fagan played a key role during the Camden Expedition, launched by Union forces, and forced the retreat of the expedition from Arkansas. He was promoted to major general on April 25, 1864, and took part in the last invasion of Missouri under General Sterling Price. He last saw combat during the battle of Mine Creek, where is division was overrun by Union forces under the command of Alfred Pleasonton. Fagan served until the end of the war as commander of the District of Arkansas. After the war he served in political posts, and returned to the life of a planter.