Lorenzo Thomas was born in New Castle, Delaware, in 1804. After graduating from West Point in 1823, he fought in the Seminole War and the Mexican War where he won a brevet.

Appointed as Chief of Staff under General Winfield Scott until 7th March, 1861. Appointed Adjutant General on the outbreak of the American Civil War, he was put in charge of organizing black regiments in March, 1863.

In 1867 President Andrew Johnson attempted to replace his Secretary of War, Edwin M. Stanton, with Ulysses S. Grant. This was a violation of the Tenure of Office Act and some members of the Republican Party began talking about impeaching Johnson. Stanton refused to go and was supported by the Senate. Grant now stood down and Thomas agreed to become the new Secretary of War. This was only a short-term measure and he soon left office.

Lorenzo Thomas died in 1875.