Born in Danbury, Connecticut, Whittlesey attended the public schools and graduated from Yale College in 1817. He then attended Litchfield Law School, was admitted to the bar in 1818 and commenced practice in Danbury, Connecticut. He served as a probate judge. He married Caroline Holley (1800–1841).
Whittlesey was elected as a Jacksonian to the Twenty-fourth Congress to fill the vacancy caused by the death of Zalmon Wildman. He was reelected as a Democrat to the Twenty-fifth Congress and served from April 29, 1836, to March 3, 1839. He was an unsuccessful candidate for reelection in 1838 to the Twenty-sixth Congress. He moved to Pheasant Branch, near Madison, Wisconsin, in 1846. He resumed practicing law and was also engaged in farming. He served as member of the Wisconsin Senate in 1853 and 1854. He died at Pheasant Branch, Wisconsin, August 20, 1868. He was interred in Forest Hill Cemetery, Madison, Wisconsin.
- United States Congress. “Thomas T. Whittlesey (id: W000433)”. Biographical Directory of the United States Congress.
- Thomas Whittlesey, Litchfield Historical Society