After studying law at Transylvania University in Lexington, Kentucky, he settled in Edwardsville, Illinois.
Four years later, he served a partial term in the Illinois House of Representatives for Madison County before being appointed Attorney General, a post he held for a single year.
His circuit included New Salem, where Thomas heard cases argued by Abraham Lincoln.
[1] When Stephen A. Douglas gave up his seat on the Illinois Supreme Court in 1843 after being elected to Congress, Governor Thomas Ford appointed Thomas as Douglas's successor.
After retiring from the Supreme Court in 1848, he moved first to Galena and then to Chicago, where he died in 1850.