After serving in the Georgia House of Representatives in 1853, 1855, and 1857, Hardeman was elected in 1858 as an Opposition Party candidate to the 36th United States Congress as a Representative of Georgia's 3rd congressional district and served a partial term from March 4, 1859, until January 23, 1861, when he resigned to become a captain in the Floyd Rifles.

During the American Civil War, Hardeman was major of the 2nd Georgia Battalion in the Confederate States Army.

In 1882, Hardeman won the election again to the U.S. House of Representatives as an at-large Democrat to the 48th United States Congress.

[5] Thomas Hardeman Jr. died in Macon, Georgia, on March 6, 1891, and was buried in that city's Rose Hill Cemetery.

[6] The Colonel Thomas Hardeman Jr. Chapter 2170 of the United Daughters of the Confederacy was named in his honor.