After graduation from Harvard, Preston was admitted to the bar and commenced practice in Louisville in 1839.

After the war, he was a delegate to the State constitutional convention in 1849 and a member of the Kentucky House of Representatives in 1850.

President James Buchanan appointed Preston as Envoy Extraordinary and Minister Plenipotentiary to Spain in 1858.

[1] Shortly thereafter, Preston was made a colonel and became volunteer aide-de-camp to his brother-in-law, Albert Sidney Johnston, who then had his Army of Central Kentucky quartered at Bowling Green.

He was appointed Envoy Extraordinary and Minister Plenipotentiary from the Confederacy to Maximilian, Emperor of Mexico in 1864.