He served as a U.S. representative and United States senator, and held leadership positions in both bodies.

Varnum became a farmer, and at age 18 received his commission as a captain in the Massachusetts militia.

Joseph Bradley Varnum was born in Dracut, Massachusetts, in Middlesex County, on January 29, 1750, or 1751.

Varnum served as President pro tempore of the U.S. Senate from December 6, 1813, to February 3, 1814, during the Thirteenth Congress.

Henry Wilson, in his History of Slavery, quotes Varnum in the debate on the bill for the government of the Mississippi Territory before the United States House of Representatives in March 1798 as having been very strong and outspoken in his opposition to Negro servitude.

On March 3, 1805, Varnum submitted a Massachusetts Proposition to amend the Constitution[note 1] and Abolish the Slave Trade.

This proposition was tabled until 1807, when under Varnum's leadership the amendment moved through Congress and passed both houses on March 2, 1807.