Washburn won election to the Massachusetts Senate in 1850 and served two years in the Massachusetts House of Representatives from 1853 to 1855. His elections to the state legislature were supported by proponents of the Hoosac Tunnel. In 1862 Washburn ran for Congress as a Republican, winning election to the 38th Congress against no opposition. He was reelected four times, winning each time by wide margins. He was viewed as relatively moderate, in comparison to the mostly Radical Republican Massachusetts delegation. He served as chairman of the Committee on Claims during the 41st Congress.