Lorenzo Thomas (October 26, 1804 – March 2, 1875) was a career U.S. Army officer who was Adjutant General of the Army during the American Civil War. After the war, he was appointed temporary Secretary of War by President Andrew Johnson, precipitating Johnson's impeachment.

Thomas was born in New Castle, Delaware. He graduated from the United States Military Academy in 1823, and was commissioned a second lieutenant in the 4th U.S. Infantry. He fought in the Seminole War in Florida and, during the Mexican-American War, he was the chief of staff to General William O. Butler. He received a brevet promotion to lieutenant colonel for Monterrey. From 1853 to 1861, he served as chief of staff to the commanding general of the U.S. Army, Winfield Scott.

Just before the start of the Civil War, Thomas was promoted to colonel and adjutant general of the U.S. Army on March 7, 1861. He was promoted to brigadier general on May 7 and he held the position of adjutant general until he retired in 1869, except for a special assignment to recruit African-American troops in the Military Division of the Mississippi from 1863 to 1865. Thomas did not get along well with Secretary of War Edwin M. Stanton and this assignment outside of Washington, D.C., was considered a form of banishment. Many historians have claimed Thomas was banished in disgrace after conspiring to defame Union General William T. Sherman as insane. From March 17 to July 23, 1862, he served as the chairman of the War Board, the organization that assisted President Abraham Lincoln and Secretary Stanton in the management of the War Department and the command of the Union armies during the period in which there was no general-in-chief. Just before the end of the war, he was given a brevet promotion to major general in the regular army, as of March 13, 1865.

On February 21, 1868, President Johnson attempted to replace Stanton by appointing Thomas as Secretary of War ad interim. Thomas, still stinging from his bad treatment by Stanton, boasted of his ability and determination to oust him from office by force, if necessary. Some historians believe that it was this attitude in his testimony at Johnson's impeachment trial in the Senate that was partially responsible for Johnson's acquittal. Thomas retired from the Army on February 22, 1869, ten days before Johnson left office. He died in Washington and is buried in Oak Hill Cemetery, Georgetown, Washington, D.C.

* Sears, Stephen W., Neely, Mark E., Fellman, Michael, and Simon, John Y., ed. by Boritt, Gabor S. Lincoln's Generals. Oxford University Press. 1994. pp. 135-138. ISBN 0195101103 * Eicher, John H., and Eicher, David J., Civil War High Commands, Stanford University Press, 2001, ISBN 0-8047-3641-3. * Sears, Stephen W., Neely, Mark E., Fellman, Michael, and Simon, John Y., ed. by Boritt, Gabor S. Lincoln's Generals. Oxford University Press. 1994. ISBN 0195101103 * Warner, Ezra J., Generals in Blue: Lives of the Union Commanders, Louisiana State University Press, 1964, ISBN 0-8071-0822-7.

* Document detailing $52,899 in property losses Thomas suffered during the Civil War. Thomas was eventually awarded $6,500 from the government.