He briefly served in the Second Seminole War and was then assigned to garrison duty along the Canada–US border from late 1837 through 1838, when he was reassigned to other military posts for the next decade.

He was engaged in the siege of Vera Cruz, and received two brevet promotions for bravery: to captain for Cerro Gordo and to major for Contreras and Churubusco.

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, Fort Zachary Taylor.

[1][2] 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 the 3rd Division of the II Corps at the Battle of Antietam, making the first attack on Confederate units in the Sunken Road.

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.