[1] Hagan served in Hays's Texas Rangers, a cavalry unit in Major General Zachary Taylor's army during the Mexican–American War.

[2] After the war, he returned to Mobile where he bought and subsequently managed a plantation instead of remaining in the family mercantile business.

[5][6] At the beginning of the Civil War, James Hagan organized and was elected captain of a cavalry company for the Alabama Militia, the "Mobile Dragoons," which served on guard duty along the Gulf Coast.

[5] Hagan led his men in a mounted charge at the Battle of Perryville which was highly commended by his brigade commander, Brigadier General Joseph Wheeler.

[5] Hagan's brigade was part of Wheeler's force which opposed Major General William T. Sherman's March to the Sea and Carolinas Campaign.

[2][5] Major General Wheeler later wrote that he had been told unofficially by Confederate States War Department officials that brigadier general commissions had been issued for Hagan, Henry Marshall Ashby and Moses Wright Hannon near the end of the war, but no such commissions ever were delivered.

[5] President Grover Cleveland appointed him crier of the United States District Court in Alabama in 1885.