He graduated from the United States Military Academy in 1843, served as a captain in the U.S. Army in the Mexican-American War, and was wounded at the Battle of Buena Vista.

[2] In 1843, French graduated from the United States Military Academy along with several future Civil War generals including Christopher C. Augur, William B. Franklin, Franklin Gardner, Ulysses S. Grant, Charles Smith Hamilton, Henry M. Judah, John J. Peck, Joseph J. Reynolds, Roswell Ripley and Frederick Steele.

[2] In August, 1845, he sailed from Baltimore with Major Samuel Ringgold's battery of horse artillery to join General Zachary Taylor and the Army of Occupation in Aransas Pass, Texas.

[6] On July 4, 1847, he was presented a ceremonial sword from the citizens of New Jersey which contained the inscription "for distinguished service in the battles of Palo Alo, Resaca de la Palma and Buena Vista".

On February 9, 1849, he was presented on resolution from the New Jersey Legislature a second sword with the inscription "For brave and gallant conduct displayed in the battles of Palo Alto, Resaca de la Palma, and Monterrey.

He was promoted to full captain on January 12, 1848 and commissioned as assistant quartermaster in the general staff of the army working under his former classmate, Rufus Ingalls.

[9] He acquired a plantation through his marriage[10] to Eliza Matilda Roberts[8] along Deer Creek near Greenville, Mississippi, and left the army to manage it.

[17] French was on medical leave in Columbus, Georgia, and Warm Springs, Arkansas, due to the lingering effects of dysentery from August 4 to October 1863.

[19] Johnston initially expressed concern to Jefferson Davis that French's Northern heritage would make it difficult for the troops to accept him.

Davis dismissed this concern and reminded Johnston that French was a wealthy plantation owner in Mississippi and served in the state militia after secession.

After Atlanta capitulated on October 5, 1864, Hood ordered French and his division to capture Altoona Pass and break the line of communications for Sherman's army.

At the Battle of Allatoona, French was unable to capture the pass, which was guarded by a federal garrison under Brigadier General John M. Corse.

The fierce fighting concluded when federal reinforcements arrived, forcing French's troops to retreat to New Hope Church and rejoin the Army of Tennessee.

French suffered an eye infection that rendered him nearly blind and he relinquished command to Claudius W. Sears before the Battle of Nashville.

In his book, he criticized Governor John J. Pettus and Confederate generals William J. Hardee, Hood and Leonidas Polk.

[12] During World War II, the Liberty ship SS Samuel G. French was built in Panama City, Florida, and named in his honor.