At the start of the Civil War, Captain French and the 1st U.S. Artillery were stationed at Eagle Pass, Texas. He refused to surrender his garrison to the Confederate-aligned state authorities as they requested. Instead, he moved his men to the mouth of the Río Grande in sixteen days and sailed to Key West, where he quartered at the Federal military post there. Shortly thereafter, he was elevated to major and assumed command of the base. In conjunction with the Union Navy, he was instrumental in shutting off Key West to slave traders.
He was promoted to brigadier general of volunteers as of September 28, 1861, and was assigned to the Army of the Potomac, where he commanded a brigade of the II Corps in the Peninsula Campaign. He was engaged at the battles of Yorktown, Seven Pines, Oak Grove, Gaines’ Mill, Garnett’s & Golding’s Farm, Savage’s Station, Glendale, and Malvern Hill. French received praise in official reports for his actions and leadership, and was promoted to command a division during the Northern Virginia Campaign.
French commanded elements of the VIII Corps and the District of Harpers Ferry during the Gettysburg Campaign, but shortly after Maj. Gen. Daniel E. Sickles was wounded at the Battle of Gettysburg, French assumed command of the battered III Corps. His military reputation was ruined during the Mine Run Campaign in November 1863 when Maj. Gen. George G. Meade claimed that French’s corps moved too slowly to exploit a potential advantage over Gen. Robert E. Lee. This engagement was the last for the III Corps, which was reorganized out of the Union Army in the spring of 1864, and French was mustered out of volunteer service on May 6, 1864.
He remained in the regular army, and for the remainder of the war, he served on military boards in Washington, D.C.. French ended the war with the regular army rank of colonel of the 4th U.S. Artillery.