Thomas studied law with his elder brother Richard Symmes Thomas (1772–1828) in
Bracken County, Kentucky, along the
Ohio River, then moved to nearby
Mason County, Kentucky, where he served as the county clerk until 1803. He then moved north of the
Ohio River to
Lawrenceburg in
Indiana Territory, where he continued to practice law and became the territorial deputy attorney general in 1805. In the same year, he began serving as a delegate to the Territorial House of Representatives, and fellow delegates chose him as their speaker from 1805 to 1808. When
Benjamin Parke resigned as the territorial delegate to Congress, Thomas was appointed to fill the vacancy from October 22, 1808, until March 3, 1809, when Thomas moved westward to the new
Illinois Territory as discussed below.
Jonathan Jennings succeeded him as Indiana's territorial delegate and would later become the new state of
Indiana's first U.S. representative and later Indiana's
Governor. Thomas moved westward to
Kaskaskia, Illinois, on the
Mississippi River (which made it flood-prone and ultimately transformed it into an island), then
Cahokia and later
Edwardsville in
Madison County, Illinois, where Thomas would later train his nephew, somewhat confusingly named
Jesse B. Thomas, Jr., who would have a distinguished career as an Illinois lawyer and judge. 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. Thomas exercised those duties from 1809 until 1818. Voters from
St. Clair County (the oldest county in Illinois and with Cahokia as its major town) elected Thomas as
Democratic-Republican to the Illinois State Constitutional Convention in 1818. 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). In 1820, Thomas proposed the
Missouri Compromise to permit slavery in Missouri while limiting slavery in the remainder of the Louisiana Purchase. In 1823 he switched parties and became a
Crawford Republican. He served as chairman on the
Committee on Public Lands in the
16th and
18th Congresses. He refused the nomination for a third term and moved to
Mount Vernon, Ohio, in 1829, where he lived the rest of his life. ==Death and legacy==