He was working as a circuit attorney when Governor William Clark appointed him attorney for the Northern District of Missouri. He served one term in the state House of Representatives and was then elected to Congress.

Bates was one of the founders of the Whig Party. He was a strong opponent of slavery and was nominated for president in 1860, but the nomination eventually went to Abraham Lincoln. Upon his election, Lincoln offered Bates a cabinet position. He chose Attorney General and became the first cabinet member from west of the Mississippi.

His brother Frederick died after only one year as Governor of Missouri and a second brother was killed in a duel. Although he and his wife had 17 children, only five lived to adulthood.

Bates also served as a vice-president for the Missouri Historical Society, which he co-founded. He died in 1869 and is buried in Bellefontaine Cemetery. [Final Resting Place, p. 40]