[1][2] Thomas was probably born in Shepherdstown, Virginia (now West Virginia), in part according to his 1850 census entry, although Appleton's Cyclopedia stated he was born somewhat east on what became the National Road in Hagerstown, Maryland (as was his slightly elder brother) and that he was descended from Lord Baltimore.

When Illinois became a territory in 1809, President James Madison (with the consent of the U.S. Senate) appointed Thomas judge of the United States court for the northwestern judicial district.

Fellow delegates elected him to preside over the convention, which chose not to accept slavery in the new state.

Upon the new state's admittance to the Union, fellow legislators elected Thomas to the U.S. Senate, in which he would serve for two terms (from 1818 until his retirement in 1829).

He refused the nomination for a third term and moved to Mount Vernon, Ohio, in 1829, where he lived the rest of his life.