Harris was commissioned a second lieutenant in the 12th U.S. Infantry Regiment[1] in 1848 but peace was declared to end the Mexican–American War on the day he reported for duty.

He then smuggled equipment for the Confederate States Army from Europe through the Union blockade of the Confederacy.

After the Civil War, Harris worked for the Life Insurance Association of America in St. Louis, Missouri, and opened an office in Texas in 1870.

After this, he moved to Kentucky, where his friendship with the governor gained him an appointment as assistant secretary of state.

Thomas Alexander Harris died on April 9, 1895, in Pewee Valley, Kentucky, near Louisville.