In 1861, Green joined the Confederate army as a colonel in the 5th Texas cavalry. He served at the Battle of Valverde in New Mexico. In January 1863, he served with distinction in the operations at Galveston, TX, for which he was rewarded with a promotion to brigadier general in May 1863. Sent to Louisiana after his promotion, he participated in the battles of Mansfield and Pleasant Hill, during the Red River campaign. He was killed during the battle of Blair's Landing in April 1864.