He served as delegate to the State constitutional convention in 1867 and was elected president of the Republican State executive board in 1867. He was elected to the South Carolina Senate in 1868, but resigned before the session to take a seat in Congress. He served as delegate to the 1868 Republican National Convention. Upon the readmission of South Carolina to the Union, Whittemore was elected as a Republican to the Fortieth and Forty-first Congresses and served from July 18, 1868, to February 24, 1870, when he resigned under the eminent threat of expulsion from the House.